Method and apparatus for transmitting downlink reference signal in wireless communication system that supports multiple antennas

ABSTRACT

A method for transmitting reference signals for eight or fewer antenna ports includes mapping a portion of common reference signals (CRSs) for four or fewer antenna ports into a downlink subframe including 1 st  slot and 2 nd  slot with a normal cyclic prefix configuration; mapping channel status information reference signals (CSI-RSs) for eight or fewer antenna ports into the downlink subframe according to a preset pattern; and transmitting the downlink subframe into which the CRSs and the CSI-RSs are mapped, wherein the preset pattern defines the CSI-RSs for eight or fewer antenna ports to be mapped onto two OFDM symbols of the data region in the downlink subframe, with the two OFDM symbols being spaced apart by one OFDM symbol, and wherein the portion of the CRSs for four or fewer antenna ports is limited to the CRSs for two or fewer antenna ports.

TECHNICAL FIELD

The following description relates to a wireless communication systemand, more particularly, to a method and apparatus for transmitting adownlink reference signal in a wireless communication system whichsupports multiple antennas.

BACKGROUND ART

A Multiple Input Multiple Output (MIMO) system refers to a system forimproving data transmission/reception efficiency using multipletransmission antennas and multiple reception antennas. MIMO technologyincludes a spatial diversity scheme and a spatial multiplexing scheme.The spatial diversity scheme is suitable for data transmission of a userequipment (UE) which moves at a high speed, because transmissionreliability is increased or a cell radius is increased through diversitygain. The spatial multiplexing scheme can increase data transfer ratewithout increasing system bandwidth by simultaneously transmittingdifferent data.

In a MIMO system, each transmission antenna has an independent datachannel. The transmission antenna may be a virtual antenna or a physicalantenna. A receiver estimates a channel with respect to eachtransmission antenna and receives data transmitted from eachtransmission antenna. Channel estimation refers to a process ofcompensating for signal distortion due to fading so as to restore thereceived signal. Fading refers to a phenomenon in which the intensity ofa signal is rapidly changed due to multi-path and time delay in awireless communication system environment. For channel estimation, areference signal known to both a transmitter and a receiver isnecessary. The reference signal may be abbreviated to RS or referred toas a pilot signal according to the standard implemented.

A downlink reference signal is a pilot signal for coherent demodulation,such as a Physical Downlink Shared Channel (PDSCH), a Physical ControlFormat Indicator Channel (PCFICH), a Physical Hybrid Indicator Channel(PHICH), and a Physical Downlink Control Channel (PDCCH). The downlinkreference signal includes a Common Reference Signal (CRS) shared amongall UEs in a cell and a Dedicated Reference Signal (DRS) for a specificUE. The CRS may be referred to as a cell-specific reference signal. TheDRS may be referred to as a UE-specific reference signal.

In a system having an antenna configuration (e.g., a system according tothe LTE-A standard supporting eight transmission antennas) developed asan extension of a legacy communication system (e.g., a system based onLTE release 8 or 9), it is necessary to transmit a reference signal foracquiring channel state information (CSI) at a reception side, that is,a CSI-RS.

DISCLOSURE Technical Problem

An object of the present invention is to provide a method for arrangingCSI-RSs, which is capable of reducing CSI-RS transmission overhead inMIMO transmission and optimizing channel estimation performance byCSI-RSs.

Technical Solution

The object of the present invention can be achieved by providing amethod for transmitting reference signals for 8 or fewer antenna portsat a base station, the method including mapping some of common referencesignals (CRSs) for four or fewer antenna ports to a downlink subframeincluding a first slot and a second slot and having a normal cyclicprefix (CP), mapping channel state information-reference signals(CSI-RSs) for the 8 or fewer antenna ports to the downlink subframeaccording to a predetermined pattern, and transmitting the downlinksubframe to which the CRSs and the CSI-RSs are mapped, wherein thepredetermined pattern defines the CSI-RSs for the 8 or fewer antennaports to be mapped to two orthogonal frequency division multiplexing(OFDM) symbols in a data region of the downlink subframe, the two OFDMsymbols being separated by one OFDM symbol, and wherein some of the CRSsfor the 4 or fewer antenna ports are limited to CRSs for 2 or fewerantenna ports.

The CRSs for the 2 or fewer antenna ports may be mapped to first andfifth OFDM symbols of the first slot and first and fifth OFDM symbols ofthe second slot.

The two OFDM symbols defined in the predetermined pattern for theCSI-RSs may be second and fourth OFDM symbols of the second slot.

The predetermined pattern may define the CSI-RSs for the 8 or fewerantenna ports to be mapped to one or more of four subcarrier locationsin each of the two OFDM symbols, and the four subcarrier locationsdefined in the predetermined pattern may include two consecutivesubcarrier locations and two other subcarrier locations separatedtherefrom by 4 subcarriers.

The four subcarrier locations defined in the predetermined pattern maybe shifted by 2 subcarriers on a per cell or cell group basis.

The two OFDM symbols may be the second and fourth OFDM symbols of thesecond slot, and the four subcarrier locations may be subcarrier indexes0, 1, 6 and 7, subcarrier indexes 2, 3, 8 and 9 or subcarrier indexes 4,5, 10 and 11.

If the number of antenna ports of the base station is 2 or 4, theCSI-RSs may be mapped to some of the locations defined in thepredetermined pattern.

The CSI-RSs for the 8 or fewer antenna ports may be grouped into a totalof four groups such that CSI-RSs for two antenna ports configure onegroup, the CSI-RSs for two antennas of each of the four groups may bemultiplexed at the same subcarrier location of the two OFDM symbolsusing a code division multiplexing (CDM) scheme, and the four groups maybe multiplexed at different subcarrier locations using a frequencydivision multiplexing (FDM) scheme.

In another aspect of the present invention, there is provided a methodof, at a user equipment, estimating a channel using channel stateinformation-reference signals (CSI-RSs) for 8 or fewer antenna portsfrom a base station, the method including receiving a downlink subframeincluding a first slot and a second slot and having a normal cyclicprefix (CP), to which some of common reference signals (CRSs) for fouror fewer antenna ports are mapped and to which channel stateinformation-reference signals (CSI-RSs) for the 8 or fewer antenna portsare mapped according to a predetermined pattern, and estimating thechannel using the CSI-RSs, wherein the predetermined pattern defines theCSI-RSs for the 8 or fewer antenna ports to be mapped to two orthogonalfrequency division multiplexing (OFDM) symbols in a data region of thedownlink subframe, the two OFDM symbols being separated by one OFDMsymbol, and wherein some of the CRSs for the 4 or fewer antenna portsare limited to CRSs for 2 or fewer antenna ports.

The CRSs for the 2 or fewer antenna ports may be mapped to first andfifth OFDM symbols of the first slot and first and fifth OFDM symbols ofthe second slot.

The two OFDM symbols defined in the predetermined pattern for theCSI-RSs may be second and fourth OFDM symbols of the second slot.

The predetermined pattern may define the CSI-RSs for the 8 or fewerantenna ports to be mapped to one or more of four subcarrier locationsin each of the two OFDM symbols, and the four subcarrier locationsdefined in the predetermined pattern may include two consecutivesubcarrier locations and two other subcarrier locations separatedtherefrom by 4 subcarriers.

The four subcarrier locations defined in the predetermined pattern maybe shifted by 2 subcarriers on a per cell or cell group basis.

The two OFDM symbols may be the second and fourth OFDM symbols of thesecond slot, and the four subcarrier locations may be subcarrier indexes0, 1, 6 and 7, subcarrier indexes 2, 3, 8 and 9 or subcarrier indexes 4,5, 10 and 11.

If the number of antenna ports of the base station is 2 or 4, theCSI-RSs may be mapped to some of the locations defined in thepredetermined pattern.

The CSI-RSs for the 8 or fewer antenna ports may be grouped into a totalof four groups such that CSI-RSs for two antenna ports configure onegroup, the CSI-RSs for two antennas of each of the four groups may bemultiplexed at the same subcarrier location of the two OFDM symbolsusing a code division multiplexing (CDM) scheme, and the four groups maybe multiplexed at different subcarrier locations using a frequencydivision multiplexing (FDM) scheme.

In another aspect of the present invention, there is provided a basestation for transmitting reference signals (RSs) for 8 or fewer antennaports, including a reception module configured to receive an uplinksignal from a user equipment, a transmission module configured totransmit a downlink signal to the user equipment, and a processorconfigured to control the base station including the reception moduleand the transmission module, wherein the processor maps some of commonreference signals (CRSs) for four or fewer antenna ports to a downlinksubframe including a first slot and a second slot and having a normalcyclic prefix (CP), maps channel state information-reference signals(CSI-RSs) for the 8 or fewer antenna ports to the downlink subframeaccording to a predetermined pattern, and controls transmission of thedownlink subframe to which the CRSs and the CSI-RSs are mapped, whereinthe predetermined pattern defines the CSI-RSs for the 8 or fewer antennaports to be mapped to two orthogonal frequency division multiplexing(OFDM) symbols in a data region of the downlink subframe, the two OFDMsymbols being separated by one OFDM symbol, and wherein some of the CRSsfor the 4 or fewer antenna ports are limited to CRSs for 2 or fewerantenna ports.

In another aspect of the present invention, there is provided a userequipment for estimating a channel using channel stateinformation-reference signals (CSI-RSs) for 8 or fewer antenna portsfrom a base station, including a reception module configured to receivea downlink signal from the base station, a transmission moduleconfigured to transmit an uplink signal to the base station, and aprocessor configured to control the base station including the receptionmodule and the transmission module, wherein the processor receives adownlink subframe including a first slot and a second slot and having anormal cyclic prefix (CP), to which some of common reference signals(CRSs) for four or fewer antenna ports are mapped and to which channelstate information-reference signals (CSI-RSs) for the 8 or fewer antennaports are mapped according to a predetermined pattern, and controlsestimation of the channel using the CSI-RSs, wherein the predeterminedpattern defines the CSI-RSs for the 8 or fewer antenna ports to bemapped to two orthogonal frequency division multiplexing (OFDM) symbolsin a data region of the downlink subframe, the two OFDM symbols beingseparated by one OFDM symbol, and wherein some of the CRSs for the 4 orfewer antenna ports are limited to CRSs for 2 or fewer antenna ports.

The general description and the following detailed description of thepresent invention are exemplary and are provided as additionaldescription of the claims.

Advantageous Effects

According to the embodiments of the present invention, it is possible toprovide a method and apparatus capable of reducing CSI-RS transmissionoverhead in MIMO transmission and optimizing channel estimationperformance by CSI-RSs.

The effects of the present invention are not limited to theabove-described effects and other effects which are not described hereinwill become apparent to those skilled in the art from the followingdescription.

DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing the structure of a transmitterincluding multiple antennas.

FIG. 2 is a diagram showing the structure of a downlink radio frame.

FIG. 3 is a diagram showing an example of a resource grid in onedownlink slot.

FIG. 4 is a diagram showing the structure of a downlink subframe.

FIG. 5 is a diagram showing resource elements (REs) to which downlinkcell-specific reference signals (RSs) are mapped in the case of a normalcyclic prefix (CP).

FIG. 6 is a diagram showing REs to which downlink cell-specificreference signals (RSs) are mapped in the case of an extended cyclicprefix (CP).

FIG. 7 is a diagram showing an example of a pattern in which CRSs andDRSs are arranged within one resource block (RB).

FIG. 8 is a diagram explaining a method of arranging CSI-RSs using anFDM, TDM and/or CDM scheme.

FIGS. 9 to 12 are diagrams showing various embodiments of a CSI-RSpattern.

FIG. 13 is a diagram showing a method of arranging CSI-RSs using an FDM,TDM and/or CDM scheme.

FIGS. 14 to 36 are diagrams showing various embodiments of a CSI-RSpattern.

FIGS. 37 and 38 are diagrams explaining frequency shifting of a CSI-RSpattern and various multiplexing methods.

FIG. 39 is a diagram showing a pattern of other RSs to be considered inorder to determine a CSI-RS pattern.

FIG. 40 is a diagram showing an embodiment of a CSI-RS pattern allocatedto one OFDM symbol.

FIG. 41 is a diagram showing an embodiment of a CSI-RS pattern allocatedto two OFDM symbols.

FIG. 42 is a diagram showing an embodiment of frequency shifting of aCSI-RS pattern.

FIGS. 43 to 46 are diagrams showing embodiments of a location of aCSI-RS pattern allocated to two OFDM symbols in a frequency domain.

FIG. 47 is a diagram showing the locations of two OFDM symbols to whichCSI-RSs are allocated in a time domain.

FIG. 48 is a diagram showing an embodiment of the location of a CSI-RSpattern allocated to two OFDM symbols in a frequency domain.

FIG. 49 is a diagram showing the locations of two OFDM symbols to whichCSI-RSs are allocated in a time domain.

FIG. 50 is a diagram showing the configuration of an exemplaryembodiment of a wireless communication system including a base stationand a UE according to the present invention.

BEST MODE

The following embodiments are proposed by combining constituentcomponents and characteristics of the present invention according to apredetermined format. The individual constituent components orcharacteristics should be considered optional factors on the conditionthat there is no additional remark. If required, the individualconstituent components or characteristics may not be combined with othercomponents or characteristics. Also, some constituent components and/orcharacteristics may be combined to implement the embodiments of thepresent invention. The order of operations to be disclosed in theembodiments of the present invention may be changed. Some components orcharacteristics of any embodiment may also be included in otherembodiments, or may be replaced with those of the other embodiments asnecessary.

The embodiments of the present invention are disclosed on the basis of adata communication relationship between a base station and a terminal.In this case, the base station is used as a terminal node of a networkvia which the base station can directly communicate with the terminal.Specific operations to be conducted by the base station in the presentinvention may also be conducted by an upper node of the base station asnecessary.

In other words, it will be obvious to those skilled in the art thatvarious operations for enabling the base station to communicate with theterminal in a network composed of several network nodes including thebase station will be conducted by the base station or other networknodes other than the base station. The term “Base Station (BS)” may bereplaced with a fixed station, Node-B, eNode-B (eNB), or an access pointas necessary. The term “relay” may be replaced with a Relay Node (RN) ora Relay Station (RS). The term “terminal” may also be replaced with aUser Equipment (UE), a Mobile Station (MS), a Mobile Subscriber Station(MSS) or a Subscriber Station (SS) as necessary.

It should be noted that specific terms disclosed in the presentinvention are proposed for the convenience of description and betterunderstanding of the present invention, and the use of these specificterms may be changed to another format within the technical scope orspirit of the present invention.

In some instances, well-known structures and devices are omitted inorder to avoid obscuring the concepts of the present invention and theimportant functions of the structures and devices are shown in blockdiagram form. The same reference numbers will be used throughout thedrawings to refer to the same or like parts.

Exemplary embodiments of the present invention are supported by standarddocuments disclosed for at least one of wireless access systemsincluding an Institute of Electrical and Electronics Engineers (IEEE)802 system, a 3^(rd) Generation Project Partnership (3GPP) system, a3GPP Long Term Evolution (LTE) system, an LTE-Advanced (LTE-A) system,and a 3GPP2 system. In particular, the steps or parts, which are notdescribed to clearly reveal the technical idea of the present invention,in the embodiments of the present invention may be supported by theabove documents. All terminology used herein may be supported by atleast one of the above-mentioned documents.

The following embodiments of the present invention can be applied to avariety of wireless access technologies, for example, CDMA (CodeDivision Multiple Access), FDMA (Frequency Division Multiple Access),TDMA (Time Division Multiple Access), OFDMA (Orthogonal FrequencyDivision Multiple Access), SC-FDMA (Single Carrier Frequency DivisionMultiple Access), and the like. The CDMA may be embodied with wireless(or radio) technology such as UTRA (Universal Terrestrial Radio Access)or CDMA2000. The TDMA may be embodied with wireless (or radio)technology such as GSM (Global System for Mobile communications)/GPRS(General Packet Radio Service)/EDGE (Enhanced Data Rates for GSMEvolution). The OFDMA may be embodied with wireless (or radio)technology such as Institute of Electrical and Electronics Engineers(IEEE)802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802-20, and E-UTRA(Evolved UTRA). The UTRA is a part of the UMTS (Universal MobileTelecommunications System). The 3GPP (3rd Generation PartnershipProject) LTE (long term evolution) is a part of the E-UMTS (EvolvedUMTS), which uses E-UTRA. The 3GPP LTE employs the OFDMA in downlink andemploys the SC-FDMA in uplink. The LTE-Advanced (LTE-A) is an evolvedversion of the 3GPP LTE. WiMAX can be explained by an IEEE 802.16e(WirelessMAN-OFDMA Reference System) and an advanced IEEE 802.16m(WirelessMAN-OFDMA Advanced System). For clarity, the followingdescription focuses on the 3GPP LTE and LTE-A system. However, thetechnical spirit of the present invention is not limited thereto.

In the following description, the term “rank” denotes the number ofpaths for independently transmitting signals, and the term “number oflayers” denotes the number of signal streams transmitted through eachpath. In general, since a transmitter transmits layers corresponding innumber to the number of ranks used for signal transmission, the rank hasthe same meaning as the number of layers unless otherwise noted.

FIG. 1 is a block diagram showing the structure of a transmitterincluding multiple antennas.

Referring to FIG. 1, a transmitter 100 includes encoders 110-1, . . . ,and 110-K, modulation mappers 120-1, . . . , and 120-K, a layer mapper130, a predecoder 140, resource element mappers 150-1, . . . , and 150-Kand OFDM signal generators 160-1, . . . , and 160-K. The transmitter 100includes Nt transmission antennas 170-1, . . . , and 170-Nt.

The encoders 110-1, . . . , and 110-K encode input data according to apredetermined coding method and generate coded data. The modulationmappers 120-1, . . . , and 120-K map the coded data to modulationsymbols representing locations on a signal constellation. A modulationscheme is not limited and may be m-phase shift keying (PSK) orm-quadrature amplitude modulation (QAM). For example, the m-PSK may beBPSK, QPSK or 8-PSK. The m-QAM may be 16-QAM, 64-QAM or 256-QAM.

The layer mapper 130 defines layers of the modulation symbols such thatthe precoder 140 distributes antenna-specific symbols into antennapaths. The layer is defined as an information path input to the precoder140. The previous information path of the precoder 140 may be referredto as a virtual antenna or layer.

The precoder 140 processes the modulation symbols using a MIMO schemeaccording to the multiple transmission antennas 170-1, . . . , and170-Nt and outputs antenna-specific symbols. The precoder 140distributes the antenna-specific symbols to the resource element mappers150-1, . . . , and 150-K of the antenna paths. Each information pathtransmitted to one antenna by the precoder 140 is referred to as astream, or may be referred to as a physical antenna.

The resource element mappers 150-1, . . . , and 150-K allocate theantenna-specific symbols to appropriate resource elements and multiplexthe antenna-specific symbols on a per-user basis. The OFDM signalgenerators 160-1, . . . , and 160-K modulate the antenna-specificsymbols using an OFDM scheme and output OFDM symbols. The OFDM signalgenerators 160-1, . . . , and 160-K may perform Inverse Fast FourierTransform (IFFT) with respect to the antenna-specific symbols and inserta cyclic prefix (CP) into time-domain symbols subjected to IFFT. The CPis a signal inserted into a guard interval in order to eliminateinter-symbol interference due to multiple paths in an OFDM transmissionscheme. The OFDM symbols are transmitted via the transmission antennas170-1, . . . , and 170-Nt.

FIG. 2 is a diagram showing the structure of a downlink radio frame.Referring to FIG. 2, a downlink radio frame includes 10 subframes, andone subframe includes two slots. The downlink radio frame may beconfigured by frequency division duplexing (FDD) or time divisionduplexing (TDD). A time required for transmitting one subframe isreferred to as a Transmission Time Interval (TTI). For example, onesubframe may have a length of 1 ms and one slot may have a length of 0.5ms. One slot may include a plurality of OFDM symbols in a time regionand include a plurality of Resource Blocks (RBs) in a frequency region.

The number of OFDM symbols included in one slot may be changed accordingto the configuration of a Cyclic Prefix (CP). The CP includes anextended CP and a normal CP. For example, if the OFDM symbols areconfigured by the normal CP, the number of OFDM symbols included in oneslot may be seven. If the OFDM symbols are configured by the extendedCP, the length of one OFDM symbol is increased, the number of OFDMsymbols included in one slot is less than that of the case of the normalCP. In the case of the extended CP, for example, the number of OFDMsymbols included in one slot may be six. If a channel state is unstable,for example, if a user equipment (UE) moves at a high speed, theextended CP may be used in order to further reduce inter-symbolinterference.

In case of using the normal CP, since one slot includes seven OFDMsymbols, one subframe includes 14 OFDM symbols. At this time, the firsttwo or three OFDM symbols of each subframe may be allocated to aPhysical Downlink Control Channel (PDCCH) and the remaining OFDM symbolsmay be allocated to a Physical Downlink Shared Channel (PDSCH).

The structure of the radio frame is only exemplary. Accordingly, thenumber of subframes included in the radio frame, the number of slotsincluded in the subframe or the number of symbols included in the slotmay be changed in various manners.

FIG. 3 is a diagram showing an example of a resource grid of onedownlink slot. OFDM symbols are configured by the normal CP. Referringto FIG. 3, the downlink slot includes a plurality of OFDM symbols in atime region and includes a plurality of RBs in a frequency region.Although one downlink slot includes seven OFDM symbols and one RBincludes 12 subcarriers, the present invention is not limited thereto.Each element of the resource grid is referred to as a Resource Element(RE). For example, a RE a(k,l) refers to a RE located at a k-thsubcarrier and an l-th OFDM symbol. In the case of the normal CP, one RBincludes 12×7 REs (in the case of the extended CP, one RB includes 12×6REs). Since an interval between subcarriers is 15 kHz, one RB includesabout 180 kHz in the frequency region. N^(DL) denotes the number of RBsincluded in the downlink slot. The value of N^(DL) is determined basedon downlink transmission bandwidth set by scheduling of a base station.

FIG. 4 is a diagram showing the structure of a downlink subframe. Amaximum of three OFDM symbols of a front portion of a first slot withinone subframe corresponds to a control region to which control channelsare allocated. The remaining OFDM symbols correspond to a data region towhich Physical Downlink Shared Channels (PDSCHs) are allocated. Thebasic transmission unit is one subframe. That is, a PDCCH and a PDSCHare allocated over two slots. Examples of the downlink control channelsused in the 3GPP LTE system include, for example, a Physical ControlFormat Indicator Channel (PCFICH), a Physical Downlink Control Channel(PDCCH), a Physical Hybrid automatic repeat request Indicator Channel(PHICH), etc. The PCFICH is transmitted at a first OFDM symbol of asubframe, and includes information about the number of OFDM symbols usedto transmit the control channel in the subframe. The PHICH includes aHARQ ACK/NACK signal as a response of uplink transmission. The controlinformation transmitted through the PDCCH is referred to as DownlinkControl Information (DCI). The DCI includes uplink or downlinkscheduling information or an uplink transmit power control command for acertain UE group. The PDCCH may include resource allocation andtransmission format of a Downlink Shared Channel (DL-SCH), resourceallocation information of an Uplink Shared Channel (UL-SCH), paginginformation of a Paging Channel (PCH), system information on the DL-SCH,resource allocation of an higher layer control message such as a RandomAccess Response (RAR) transmitted on the PDSCH, a set of transmit powercontrol commands for an individual UEs in a certain UE group, transmitpower control information, activation of Voice over IP (VoIP), etc. Aplurality of PDCCHs may be transmitted within the control region. Aterminal may monitor the plurality of PDCCHs. The PDCCHs are transmittedon an aggregation of one or several consecutive control channel elements(CCEs). The CCE is a logical allocation unit used to provide the PDCCHsat a coding rate based on the state of a radio channel. The CCEcorresponds to a plurality of resource element groups. The format of thePDCCH and the number of available bits are determined based on acorrelation between the number of CCEs and the coding rate provided bythe CCEs. The base station determines a PDCCH format according to a DCIto be transmitted to the terminal, and attaches a Cyclic RedundancyCheck (CRC) to control information. The CRC is masked with a RadioNetwork Temporary Identifier (RNTI) according to an owner or usage ofthe PDCCH. If the PDCCH is for a specific terminal, a cell-RNTI (C-RNTI)of the terminal may be masked to the CRC. Alternatively, if the PDCCH isfor a paging message, a paging indicator identifier (P-RNTI) may bemasked to the CRC. If the PDCCH is for system information (morespecifically, a system information block (SIB)), a system informationidentifier and a system information RNTI (SI-RNTI) may be masked to theCRC. To indicate a random access response that is a response fortransmission of a random access preamble of the terminal, a randomaccess-RNTI (RA-RNTI) may be masked to the CRC.

Common Reference Signal (CRS)

A pattern in which cell-specific reference signals, that is, commonreference signals (CRSs) are arranged on resource blocks will bedescribed with reference to FIGS. 5 and 6.

The CRS is used to estimate the channel of a physical antenna port, maybe commonly used by all terminals (UEs) located in a cell, and isdistributed over the entire band. The CRS may be used for acquisition ofchannel state information (CSI) and data demodulation.

Various CRSs may be defined according to the antenna configuration of atransmission side (base station). A 3GPP LTE (release-8) system supportsvarious antenna configurations and a downlink signal transmission side(base station) has three types of antenna configurations such as asingle antenna, two transmission antennas and four transmissionantennas. If the base station performs transmission using a singleantenna, RSs for the single antenna port are arranged. If the basestation performs transmission using two antennas, RSs for the twoantenna ports are arranged using a time division multiplexing (TDM)scheme and/or a frequency division multiplexing (FDM) scheme. That is,the RSs for the two antenna ports may be arranged on different timeresources and/or different frequency resources to be distinguished fromeach other. If the base station performs transmission using fourantennas, RSs for the four antenna ports are arranged using a TDM schemeand/or an FDM scheme. Channel information estimated by a downlink signalreception side (UE) through the CRS may be used to demodulate datatransmitted using transmission methods such as single antennatransmission, transmit diversity, closed-loop spatial multiplexing,open-loop spatial multiplexing, and multi-user MIMO (MU-MIMO).

In the case in which multiple antennas are supported, if an RS istransmitted via a certain antenna port, the RS is transmitted at thelocation of a resource element (RE) specified according to the RSpattern and no signal is transmitted at the location of the RE specifiedfor another antenna port.

In order to enhance channel estimation performance through the CRS, thelocation of the CRS in the frequency domain may be shifted on a per cellbasis to be different from each other. For example, if the RSs arelocated at every third subcarrier, the CRS may be located on a 3k-thsubcarrier in a certain cell and the CRS may be located on a (3k+1)-thsubcarrier in another cell. From the viewpoint of one antenna port, theRSs may be arranged at an interval of 6 REs (that is, an interval of 6subcarriers) in the frequency domain, and an interval between an RE inwhich an RS for one antenna port is arranged and an RE in which an RSfor another antenna port is arranged is three REs.

In the time domain, the RSs are arranged from a first OFDM symbol(symbol index 0) of each slot as a start point at a predeterminedinterval. A time interval is differently defined according to CP length.In the case of the normal CP, the RSs are located at first and fifthOFDM symbols (symbol indexes 0 and 4) of the slot and, in the case ofthe extended CP, the RSs are located at first and fourth OFDM symbols(symbol indexes 0 and 3) of the slot. In one OFDM symbol, only RSs for amaximum of two antenna ports are defined. Accordingly, for 4-Tx antennatransmission, RSs for antenna ports 0 and 1 are located at first andfifth OFDM symbols of the slot (first and fourth OFDM symbols in thecase of the extended CP) and the RSs for antenna ports 2 and 3 arelocated at a second OFDM symbol of the slot. The frequency locations ofthe RSs for the antenna ports 2 and 3 are switched in a second slot.

For the above-described locations of the CRSs, refer to FIGS. 5 and 6.More specifically, FIG. 5 shows REs to which CRSs are mapped in the caseof the normal CP. In FIG. 5, a horizontal axis denotes a time domain anda vertical axis denotes a frequency domain. In FIG. 5, the mapping unitof the RE corresponds to OFDM symbols configuring one subframe (that is,two slots) in the time domain and corresponds to subcarriers configuringone RB in the frequency domain. A smallest rectangle in thetime-frequency domain shown in FIG. 5 corresponds to one OFDM symbol inthe time domain and corresponds to one subcarrier in the frequencydomain, that is, corresponds to one RE. That is, the REs to which theRSs are mapped may be represented based on a pair of two RBs, which areconsecutive in the time domain, of one subframe including 14 OFDMsymbols×12 subcarriers in the frequency domain.

R0 to R3 shown in FIG. 5 indicate REs to which CRSs for antenna ports 0to 3 are mapped. That is, Rp indicates an RE to which RS is mapped to anantenna port index p. As described above, in the case of two antennaports and four antenna ports, an RE to which the RS of one antenna portis mapped within one slot is not used for transmission of anotherantenna port within that slot.

FIG. 6 shows REs to which CRSs for antenna ports 0 to 3 are mapped inthe case of the extended CP. In the case of the extended CP, since onesubframe includes 12 OFDM symbols, the mapping unit of the REs isrepresented by 12 OFDM symbols×12 subcarriers in FIG. 9.

In order to support spectral efficiency higher than that of the 3GPP LTE(release-8) system, a system (e.g., an LTE-A system) having the extendedantenna configuration may be designed. The extended antennaconfiguration may have, for example, eight transmission antennas. In thesystem having the extended antenna configuration, it is necessary tosupport UEs which operate in the existing antenna configuration, thatis, backward compatibility. Accordingly, it is necessary to support anRS pattern according to the existing antenna configuration and to designa new RS pattern for an additional antenna configuration. If CRSs forthe new antenna ports are added to the system having the existingantenna configuration, RS overhead is rapidly increased and thus datatransfer rate is reduced. In consideration of these problems, separateRSs for measuring the CSI for the new antenna ports need to be designed,which will be described in detail after describing a dedicated referencesignal (DRS).

Dedicated Reference Signal (DRS)

In the system having the extended antenna configuration in order toreduce RS overhead, a UE-specific RS, that is, a DRS, may be consideredin order to support data transmission via the added antennas.

In the design of the DRS for the new antenna port, a CRS pattern,frequency shifting of the CRS and power boosting need to be considered.More specifically, in order to enhance channel estimation performance bythe CRS, frequency shifting of the CRS and power boosting areconsidered. Frequency shifting means that the start point of the CRS isdifferently set on a per cell basis as described above. Power boostingmeans that the RSs are transmitted using higher power by getting thepowers of the REs except for the REs allocated for the RSs among the REsof one OFDM symbol. The DRS may be designed to have a frequency intervaldifferent from that of the CRS. If the CRS and the DRS are present inthe same OFDM symbol, the locations of the CRS and the DRS may overlapaccording to the frequency shifting of the CRS and power boosting of theCRS may adversely affect DRS transmission.

Accordingly, in the case in which the DRS and the CRS are designed tohave different frequency intervals, the two RSs are preferably designedto be located at different OFDM symbols. More specifically, the CRSs arelocated at first, second, fifth, eighth, ninth and twelfth OFDM symbolsin the case of the normal CP and are located at first, second, fourth,seventh, eighth and tenth OFDM symbols in the extended CP.

Since the DRS is an RS for data demodulation, the DRS is located in aregion to which a data channel is allocated. First to third OFDM symbols(or first and second OFDM symbols) of one subframe are used for a PDCCH(control channel) and second (or third) to last OFDM symbols areallocated for a PDSCH (data channel).

Thus, if the DRSs are allocated to locations to which the CRSs are notallocated in the region to which the data channel is allocated, the DRSsmay be arranged at the following OFDM symbol locations.

In the case of the normal CP: (third), fourth, sixth, seventh, tenth,eleventh, thirteenth and fourteenth OFDM symbols

In the case of the extended CP: (third), fifth, sixth, ninth, eleventhand twelfth OFDM symbols

Channel estimation information of an RE to which the DRS is notallocated may be acquired from channel estimation information ofneighboring REs, to which the DRSs are allocated, by an interpolationmethod. In consideration of interpolation in the time domain, the RSsare preferably located at both ends of the data channel, that is, fourthand fourteenth OFDM symbols (in the case of the extended CP, fifth andtwelfth OFDM symbols) of one subframe. However, even when the channelvaries with time by a Doppler effect due to movement of the UE, thechannel is not significantly changed within one to two OFDM symbols.Thus, although the DRSs are located inside the OFDM symbols located atboth ends of the data channel, data transfer performance according tochannel estimation is not significantly changed.

In consideration of multiple-antenna transmission, RSs for the antennaports (or layers) may be multiplexed using a TDM, FDM and/or codedivision multiplexing (CDM) scheme. That is, the RSs for the antennaports are arranged on different time resources or frequency resources tobe distinguished from each other. Alternatively, even when the RSs forthe antenna ports may be arranged on the same time resources orfrequency resources, the RSs may be distinguished from each other usingdifferent code resources.

In multiple-antenna transmission, the RSs for the antenna ports may betransmitted using CDM or TDM in adjacent OFDM symbols. If CDMtransmission is performed using two OFDM symbols, the two OFDM symbolsmay be adjacent to each other and the two OFDM symbols preferably havean interval of a maximum of one OFDM symbol therebetween if they are notadjacent to each other. If CDM transmission is performed using four OFDMsymbols, the channel must not change with time. In this case, CDMtransmission may be performed using the RSs present at different OFDMsymbols and the same frequency location.

Matters which are considered when the DRSs are allocated in thefrequency domain will now be described. First, when the DRSs areallocated in the frequency domain, the DRSs are preferably located atedges of allocated resources to enhance channel estimation performance.A 90% coherent bandwidth B_(C,90) in the frequency domain may bedetermined by Equation 1 and a 50% coherent bandwidth B_(C,50) may bedetermined by Equation 2.

B _(C,90)≈1/(50σ_(τ))  Equation 1

B _(C,90)≈1/(5σ_(τ))  Equation 2

In Equations 1 and 2, σ_(τ) denotes a root mean square (RMS) of delayspread.

In an extended typical urban (eTU) channel environment, σ_(τ) is about0.5 μs. According to Equation 1, the 90% coherent bandwidth becomesabout 10 kHz and, according to Equation 2, the 50% coherent bandwidthbecomes about 100 kHz. Since the frequency bandwidth of one RE is 15kHz, the 90% coherent bandwidth has an interval of about 1 RE and the50% coherent bandwidth has an interval of about 6 REs. Accordingly, inorder to perform interpolation of the RS in channel estimation, theinterval between the RSs is preferably less than 6 REs in the frequencydomain. In order to perform extrapolation, the interval between the RSsis preferably 1 RE.

When one RB is a minimum data transfer unit, in consideration that theDRSs are uniformly distributed in 12 REs in the frequency domain of oneRB, a structure in which RSs are arranged at both ends of the RB and anRS is arranged on a middle portion of the RB may be used. For example,in one OFDM symbol, the RSs may be located at first, sixth and eleventhREs (or second, seventh and twelfth REs) in the frequency domain. Such astructure is advantageous in that the RSs can be efficiently used andinterpolation can be efficiently performed. Since the twelfth (or first)RE is within the 90% coherent bandwidth with the eleventh (or second)RE, performance is not significantly changed even when an extrapolatedchannel or a channel of a neighboring RE is copied and used.

Meanwhile, the DRS is transmitted using the same weight as a precodingweight used for data transmission and the density of RSs may be changedaccording to the number of transmission layers (antenna ports).

FIG. 7 is a diagram showing an example of a pattern in which CRSs andDRSs are arranged within one RB (14 OFDM symbols×12 subcarriers) basedon the above-described design criterion.

Although all the CRSs for the antenna port indexes 0 to 3 are shown inFIG. 7, some of the antenna ports may be used. For example, only CRSsfor the antenna port indexes 0 to 1 (two transmission antennas) may beused or only CRSs for the antenna port index 0 may be used.

In FIG. 7( a), the DRSs are arranged on 12 REs within one RB. In FIG. 7(b), the DRSs are arranged on 24 REs within one RB. The DRS pattern shownin FIG. 7( b) may be used when the number of transmission layers isincreased. For example, the DRS pattern shown in FIG. 7( a) may be usedwhen the number of transmission layers is 1 to 2 and the DRS patternshown in FIG. 7( b) may be used when the number of transmission layersis 3 to 8. However, the present invention is not limited thereto and asuitable DRS pattern may be selected according to the number oftransmission layers.

Channel State Information Reference Signal (CSI-RS)

In a system (e.g., an LTE-A system supporting eight transmissionantennas) developed as an extension of a legacy communication system(e.g., an LTE release 8 system supporting four transmission antennas),it is necessary to transmit new RSs for acquiring CSI. Since theabove-described CRSs are RSs for the antenna ports 0 to 3, it isnecessary to additionally design new RSs for acquiring the channelstates of the extended antenna ports.

In the case of channel information for acquiring the CSI, as compared tochannel information required for data demodulation, the CSI may beacquired even when accuracy of channel estimation through RSs is low.Accordingly, the CSI-RS designed for the purpose of acquiring the CSImay be designed with density relatively lower than that of the existingRSs. For example, the CSI-RSs may be transmitted with a duty cycle suchas 2 ms, 5 ms, 10 ms or 40 ms in the time domain and RSs may betransmitted at an interval of 6 REs or 12 REs in the frequency domain.The duty cycle indicates a time unit for acquiring all the RSs for theantennas used for transmission. In addition, the CSI-RS may betransmitted over the entire frequency band.

In order to reduce overhead of the CSI-RS transmitted in one subframe,the RSs for the antenna ports may be transmitted on different subframes.However, the CSI-RSs which can support all antenna ports according tothe extended transmission antennas within the duty cycle should betransmitted. For example, if CSI-RSs supporting eight antenna ports arepresent, CSI-RSs for four antenna ports may be transmitted on a firstsubframe and CSI-RSs for the remaining antenna ports may be transmittedon a second subframe. At this time, the first and second subframes maybe subframes which are consecutive in the time domain or subframes witha certain time interval (a value smaller than the duty cycle).

Hereinafter, various embodiments of the present invention of the CSI-RSpattern will be described.

Embodiment 1

According to Embodiment 1, CSI-RSs may be located on OFDM symbols onwhich the CRSs are arranged. More specifically, CRSs may be located onfirst, second, fifth, eighth, ninth and twelfth OFDM symbols in the caseof the normal CP and may be located on first, second, fourth, seventh,eighth and tenth OFDM symbols in the case of the extended CP. In CSI-RSarrangement, except for first to third OFDM symbols on which the controlchannel (PDCCH) is located, the CSI-RSs may be located on fifth, eighth,ninth and twelfth OFDM symbols in the case of the normal CP and may belocated on fourth, seventh, eighth and tenth OFDM symbols in the case ofthe extended CP.

The CSI-RS located within one subframe may be designed to be separatedfrom the existing CRS by the same frequency interval (that is, aninterval of 3 REs).

More specifically, REs on which the CSI-RSs are arranged may be arrangedat the same interval in the frequency domain. From the viewpoint of oneantenna port, the CSI-RSs are arranged at an interval of 6 REs (that is,an interval of 6 subcarriers) in the frequency domain and the REs, onwhich RSs for one antenna port are arranged, may be arranged to beseparated from the REs, on which RSs for another antenna port arearranged, by an interval of 3 REs. In this case, the CSI-RSs may betransmitted using REs other than REs, on which the CRSs are arranged, inthe OFDM symbol on which the CRSs are located. RSs are located in oneOFDM symbol, on which the CRSs are located, at an interval of 3 REs andtwo REs for data are present between the REs for CRSs. Some of the REsfor data in the OFDM symbol on which the CRSs are located may be used asREs for CSI-RSs.

In one RB (14 OFDM symbols×12 subcarriers in the case of the normal CPor 12 OFDM symbols×12 subcarriers in the case of the extended CP), eightREs may be used for CSI-RSs. Two OFDM symbols may be used in one RB andCSI-RSs may be arranged on four REs in one OFDM symbol.

Hereinafter, a method of arranging CSI-RSs using an FDM, TDM and/or CDMscheme will be described with reference to FIG. 8. The left drawings ofFIGS. 8( a) to 8(c) show CSI-RS patterns in the case of the normal CPand the right drawings thereof show CSI-RS patterns in the case of theextended CP. The locations of the CSI-RSs shown in FIGS. 8( a) to 8(c)are exemplary and the present invention is not limited thereto. Thedescription of FIG. 8 is equally applicable to modified examples of theCSI-RS patterns of FIGS. 9 to 12.

As shown in FIG. 8( a), when four REs among 12 REs on a first OFDMsymbol are used for CSI-RSs, two REs may be used for an antenna portindex 0 (C0) and the remaining two REs may be used for an antenna portindex 1 (C1). When four REs among 12 REs on a second OFDM symbol areused for CSI-RSs, two REs may be used for an antenna port index 2 (C2)and the remaining two REs may be used for an antenna port index 3 (C3).At this time, the CSI-RSs may be arranged at the same subcarrierlocations in the two OFDM symbols (first and second OFDM symbols) onwhich the CSI-RSs are arranged. In such a CSI-RS pattern, the CSI-RSs(C0 and C1) for the antenna port indexes 0 and 1 are distinguished usingthe FDM scheme and the CSI-RSs (C2 and C3) for the antenna port indexes2 and 3 are distinguished using the FDM scheme. The CSI-RSs (C0 and C2)for the antenna port indexes 0 and 2 are distinguished using the TDMscheme and the CSI-RSs (C1 and C3) for the antenna port indexes 1 and 3are distinguished using the TDM scheme.

As shown in FIG. 8( b), when four REs among 12 REs on a first OFDMsymbol are used for CSI-RSs, four REs may be used for antenna portindexes 0 to 3. When four REs among 12 REs on a second OFDM symbol areused for CSI-RSs, four REs may be used for antenna port indexes 0 to 3.CSI-RSs may be arranged at the same subcarrier locations in the two OFDMsymbols (first and second OFDM symbols) on which the CSI-RSs arearranged. At this time, when a certain antenna port is defined in an REallocated for the CSI-RS, the RE of the first OFDM symbol and the RE ofthe second OFDM symbol located at the same subcarrier location may bedefined for different antenna ports. For example, if four REs allocatedfor CSI-RSs are sequentially allocated for antenna port indexes 0, 1, 2and 3 within 12 REs of the first OFDM symbol (C0, C1, C2 and C3), fourREs of the second OFDM symbol located at the same subcarrier locationsmay be sequentially allocated for the antenna port indexes 2, 3, 0 and 1(C2, C3, C0 and C1). In such a CSI-RS pattern, the four antenna ports onone OFDM symbol may be distinguished using the FDM scheme.Alternatively, since the CSI-RSs for different antenna ports arearranged on different OFDM symbols, the CSI-RSs may be distinguishedusing the TDM scheme.

As shown in FIG. 8( c), RSs for two antenna ports may be multiplexedusing the CDM scheme over two OFDM symbols at the same subcarrierlocation. That is, the CSI-RS (C0) for the antenna port index 0 may bearranged over two REs which are contiguous in the time domain, theCSI-RS (C1) for the antenna port index 1 may be arranged on the same RE,and C0 and C1 may be multiplexed using different code resources (e.g.,an orthogonal cover code (OCC) having a length of 2). Similarly, theCSI-RS (C2) for the antenna port index 2 may be arranged over two REswhich are contiguous in the time domain, the CSI-RS (C3) for the antennaport index 3 may be arranged on the same RE, and C2 and C3 may bemultiplexed using different code resources. In such a method, theCSI-RSs arranged over the time domain are distinguished using orthogonalcode resources. This method may be referred to as CDM-T multiplexing.

The method of multiplexing the CSI-RSs is not limited to the methodsshown in FIGS. 8( a) to 8(c) and may be implemented by various methodssuch as TDM, FDM and/or CDM.

FIGS. 9 to 12 show various embodiments according to the above-describedmethods of arranging CSI-RSs. The left drawings of FIGS. 9 to 12 showCSI-RS patterns in the case of the normal CP and the right drawingsthereof show CSI-RS patterns in the case of the extended CP.

Although all the CRSs for antenna port indexes 0 to 3 are shown in FIGS.9 to 12, some of the antenna ports may be used. For example, only CRSsfor the antenna port indexes 0 to 1 (two transmission antennas) may beused or only CRSs for the antenna port index 0 (single transmissionantenna) may be used.

In FIGS. 9 to 12 associated with Embodiment 1, as described above,CSI-RSs are arranged on two OFDM symbols among OFDM symbols on whichCRSs are arranged. That is, the CSI-RSs may be located on two OFDMsymbols among fifth, eighth, ninth and twelfth OFDM symbols in the caseof the normal CP and may be located on two OFDM symbols of fourth,seventh, eighth and tenth OFDM symbols in the case of the extended CP.

More specifically, in FIG. 9( a), CSI-RSs are located on fifth andeighth OFDM symbols in the case of the normal CP and are located onfourth and seventh OFDM symbols in the case of the extended CP. In FIG.9( b), CSI-RSs are located on fifth and ninth OFDM symbols in the caseof the normal CP and are located on fourth and eighth OFDM symbols inthe case of the extended CP. In FIG. 9( c), CSI-RSs are located on fifthand twelfth OFDM symbols in the case of the normal CP and are located onfourth and tenth OFDM symbols in the case of the extended CP. In FIG.10( a), CSI-RSs are located on eighth and ninth OFDM symbols in the caseof the normal CP and are located on seventh and eighth OFDM symbols inthe case of the extended CP. In FIG. 10( b), CSI-RSs are located oneighth and twelfth OFDM symbols in the case of the normal CP and arelocated on seventh and tenth OFDM symbols in the case of the extendedCP. In FIG. 10( c), CSI-RSs are located on ninth and twelfth OFDMsymbols in the case of the normal CP and are located on eighth and tenthOFDM symbols in the case of the extended CP.

In the embodiments of FIGS. 9 to 10, patterns in which the CSI-RSs arearranged on second, fifth, eighth and twelfth subcarriers in one OFDMsymbol are shown.

OFDM symbols in which CSI-RSs are arranged in FIGS. 11( a), 11(b),11(c), 12(a), 12(b) and 12(c) correspond to OFDM symbols in whichCSI-RSs are arranged in FIGS. 9( a), 9(b), 9(c), 10(a), 10(b) and 10(c),except that the locations of subcarriers on which the CSI-RSs arearranged are different on each OFDM symbol. That is, in the embodimentsof FIGS. 11 to 12, patterns in which CSI-RSs are arranged on first,fourth, seventh and tenth subcarriers in one OFDM symbol are shown.

Embodiment 2

According to Embodiment 2, CSI-RSs may be located on OFDM symbols onwhich the CRSs are not arranged. In the OFDM symbols on which the CRSsare not arranged, an OFDM symbol on which the DRS is located and an OFDMsymbol on which only a data signal is located are present. In the casein which a subcarrier interval of a DRS is designed to be different froma subcarrier interval of a CSI-RS, if the DRS and the CSI-RS are locatedon the same OFDM symbol, collision therebetween may occur. Since RSs aretransmitted with power higher than that of data, collision between RSssignificantly decreases channel estimation performance using RSs ascompared to collision between an RS and data. In the case in whichCSI-RSs are arranged on an OFDM symbol on which a DRS is not arrangedbut only a data signal is located, even if collision between the CSI-RSand the data occurs, no problem occurs in channel estimation of areception side using the CSI-RS. In consideration of this point, variousCSI-RS arrangement patterns may be designed according to DRS arrangementpatterns.

In the case of the normal CP, OFDM symbols on which CRSs are notarranged in one RB (14 OFDM symbols×12 subcarriers) include third,fourth, sixth, seventh, tenth, eleventh, thirteenth and fourteenth OFDMsymbols. In the case of the extended CP, OFDM symbols on which CRSs arenot arranged in one RB (12 OFDM symbols×12 subcarriers) include third,fifth, sixth, ninth, eleventh and twelfth OFDM symbols. In addition, thecontrol channel (PDCCH) may be allocated to first to second (or third)OFDM symbols and the CSI-RSs are designed so as not to be arranged onthese OFDM symbols. In consideration of the OFDM symbols on which theDRSs are located, for example, if a DRS pattern shown in FIG. 7 is used,the DRSs may be arranged on sixth, seventh, thirteenth and fourteenthOFDM symbols in the case of the normal CP. Similarly, the DRSs may bearranged on fifth, sixth, eleventh and twelfth OFDM symbols in the caseof the extended CP. Accordingly, if the CSI-RSs are arranged on the OFDMsymbols on which the DRSs are not located but only data signals arelocated, the CSI-RSs may be located on (third), fourth, tenth andeleventh OFDM symbols in the case of the normal CP and may be located on(third) ninth OFDM symbols in the case of the extended CP.

As described above, although the CSI-RSs may be arranged on the OFDMsymbols on which the CRSs are not located, in case of using the CRSs forfour transmission antennas, some of the REs allocated for the CRSs maybe used for the CSI-RSs.

For example, in the second slot of one subframe, CSI-RSs may be arrangedat RE locations (R2 and R3 of FIGS. 5 and 6) allocated for the CRSs inthe OFDM symbol (the ninth OFDM symbol in the case of the normal CP andthe eighth OFDM symbol in the case of the extended CP) on which the CRSsfor the antenna port indexes 2 and 3 are located. This may be calledreuse of CRS REs for the antenna ports 2 and 3.

In this case, if the CSI-RSs are arranged at the locations of the CRSsfor the antenna port indexes 2 and 3 in the case of the extended CP,ambiguity may occur in analysis of the RSs in the legacy UE (e.g., theUE according to LTE release 8 or 9) which cannot analyze the CSI-RS.Accordingly, in the case of the normal CP, CRSs configured for a maximumof four transmission antenna ports may be specified to recognize asingle transmission antenna, two transmission antennas and fourtransmission antennas. In contrast, in the case of the extended CP, onlyCRSs configured for a maximum of two transmission antenna ports may bespecified to recognize only a single transmission antenna and twotransmission antennas.

In a first slot of one subframe, CSI-RSs may be arranged at the RElocations allocated for the CRSs in the OFDM symbol (the second OFDMsymbol) to which the CRSs for the antenna ports 2 and 3 are allocated.

Alternatively, CSI-RSs may be arranged at the RE locations allocated forthe CRSs in one of the OFDM symbols (the first, fifth, eighth andtwelfth OFDM symbols of one subframe in the case of the normal CP) towhich the CRSs for the antenna port indexes 0 and 1 are allocated.

The OFDM symbols on which the CRI-RSs are arranged may be determinedusing the above-described method and the CSI-RSs may be arranged on twoOFDM symbols of one subframe. The CSI-RSs may be arranged at the sameinterval in the frequency domain. From the viewpoint of one antennaport, the RSs may be arranged at an interval of 6 REs (that is, aninterval of six subcarriers) in the frequency domain and the RE, onwhich the RSs for one antenna port are arranged, may be arranged to beseparated from the RE, on which the RSs for another antenna port arearranged, by an interval of 3 REs in the frequency domain.

In one RB (14 OFDM symbols×12 subcarriers in the case of the normal CPor 12 OFDM symbols×12 subcarriers in the case of the extended CP), eightREs may be used for CSI-RSs. Two OFDM symbols may be used in one RB andCSI-RSs may be arranged on four REs in one OFDM symbol.

Hereinafter, a method of arranging CSI-RSs using an FDM, TDM and/or CDMscheme will be described with reference to FIG. 13. The left drawings ofFIGS. 13( a) to 13(c) show CSI-RS patterns in the case of the normal CPand the right drawings thereof show CSI-RS patterns in the case of theextended CP. The locations of the CSI-RSs shown in FIGS. 13( a) to 13(c)are exemplary and the present invention is not limited thereto. Thedescription of FIG. 13 is equally applicable to modified examples of theCSI-RS patterns of FIGS. 14 to 36.

As shown in FIG. 13( a), when four REs among 12 REs on a first OFDMsymbol are used for CSI-RSs, two REs may be used for an antenna portindex 0 (C0) and the remaining two REs may be used for an antenna portindex 1 (C1). When four REs among 12 REs on a second OFDM symbol areused for CSI-RSs, two REs may be used for an antenna port index 2 (C2)and the remaining two REs may be used for an antenna port index 3 (C3).At this time, the CSI-RSs may be arranged at the same subcarrierlocations in the two OFDM symbols (first and second OFDM symbols) onwhich the CSI-RSs are arranged. In such a CSI-RS pattern, the CSI-RSs(C0 and C1) for the antenna port indexes 0 and 1 are distinguished usingthe FDM scheme and the CSI-RSs (C2 and C3) for the antenna port indexes2 and 3 are distinguished using the FDM scheme. The CSI-RSs (C0 and C2)for the antenna port indexes 0 and 2 are distinguished using the TDMscheme and the CSI-RSs (C1 and C3) for the antenna port indexes 1 and 3are distinguished using the TDM scheme.

As shown in FIG. 13( b), when four REs among 12 REs on a first OFDMsymbol are used for CSI-RSs, four REs may be used for antenna portindexes 0 to 3. When four REs of 12 REs on a second OFDM symbol are usedfor CSI-RSs, four REs may be used for antenna port indexes 0 to 3.CSI-RSs may be arranged at the same subcarrier locations in the two OFDMsymbols (first and second OFDM symbols) on which the CSI-RSs arearranged. At this time, when a certain antenna port is defined in an REallocated for the CSI-RS, the RE of the first OFDM symbol and the RE ofthe second OFDM symbol located at the same subcarrier location may bedefined for different antenna ports. For example, if four REs allocatedfor CSI-RSs are sequentially allocated for antenna port indexes 0, 1, 2and 3 within 12 REs of the first OFDM symbol (C0, C1, C2 and C3), fourREs of the second OFDM symbol located at the same subcarrier locationsmay be sequentially allocated for the antenna port indexes 2, 3, 0 and 1(C2, C3, C0 and C1). In such a CSI-RS pattern, the four antenna ports onone OFDM symbol may be distinguished using the FDM scheme.Alternatively, since the CSI-RSs for different antenna ports arearranged on different OFDM symbols, the CSI-RSs may be distinguishedusing the TDM scheme.

As shown in FIG. 13( c), RSs for two antenna ports may be multiplexedusing the CDM scheme over two OFDM symbols at the same subcarrierlocation. That is, the CSI-RS (C0) for the antenna port index 0 may bearranged over two REs located on different OFDM symbols in the samesubcarrier, the CSI-RS (C1) for antenna port index 1 may be arranged onthe same RE, and C0 and C1 may be multiplexed using different coderesources (e.g., an OCC having a length of 2). Similarly, the CSI-RS(C2) for the antenna port index 2 may be arranged over two REs locatedon different OFDM symbols in the same subcarrier, the CSI-RS (C3) forthe antenna port index 3 may be arranged on the same RE, and C2 and C3may be multiplexed using different code resources. In such a method, theCSI-RSs arranged over the time domain are distinguished using orthogonalcode resources. This method may be called a CDM-T multiplexing method.

FIG. 13( d) shows a pattern in which CSI-RSs for eight antenna ports maybe multiplexed using the CDM scheme. The CSI-RS (C0) for the antennaport index 0 may be arranged over two REs located on different OFDMsymbols in the same subcarrier, the CSI-RS (C1) for the antenna portindex 1 may be arranged on the same RE, and C0 and C1 may be multiplexedusing different code resources (e.g., an OCC having a length of 2). Inaddition, the CSI-RS (C2) for antenna port index 2 may be arranged overtwo REs located on different OFDM symbols in the same subcarrier, theCSI-RS (C3) for the antenna port index 3 may be arranged on the same RE,and C2 and C3 may be multiplexed using different code resources. Inaddition, the CSI-RS (C4) for the antenna port index 4 may be arrangedover two REs located on different OFDM symbols in the same subcarrier,the CSI-RS (C5) for the antenna port index 5 may be arranged on the sameRE, and C4 and C5 may be multiplexed using different code resources. Inaddition, the CSI-RS (C6) for the antenna port index 6 may be arrangedover two REs located on different OFDM symbols in the same subcarrier,the CSI-RS (C7) for the antenna port index 7 may be arranged on the sameRE, and C6 and C7 may be multiplexed using different code resources.

The method of multiplexing the CSI-RSs is not limited to the methodsshown in FIGS. 13( a) to 13(c) and may be implemented by various methodssuch as TDM, FDM and/or CDM.

FIGS. 14 to 36 show various embodiments according to the above-describedmethods of arranging CSI-RSs. The left drawings of FIGS. 14 to 36 showCSI-RS patterns in the case of the normal CP and the right drawingsthereof show CSI-RS patterns in the case of the extended CP.

Although all the CRSs for antenna port indexes 0 to 3 are shown in FIGS.14 to 36, some of the antenna ports may be used. For example, only CRSsfor the antenna port indexes 0 to 1 (two transmission antennas) may beused or only CRSs for the antenna port index 0 (single transmissionantenna) may be used.

The CSI-RS patterns shown in FIGS. 14 to 36 may be frequency-shifted.Frequency shifting may be performed on a per cell basis. That is, thelocations of the CRI-RSs may be shifted in the frequency domain on a percell basis. For example, if the CSI-RSs are located at every thirdsubcarrier, a first cell may be arranged on a 3k-th subcarrier, a secondcell may be arranged on a (3k+1)-th subcarrier, and a third cell may bearranged on a (3k+2)-th subcarrier. The CSI-RS and the CRS may befrequency-shifted with the same offset.

If the CSI-RSs are located at the same subcarrier locations as thesubcarrier, on which the CRSs are located, in the CSI-RS patterns shownin FIGS. 14 to 36, the CSI-RSs may be defined at the locations shiftedby one subcarrier or two subcarriers.

Hereinafter, the CSI-RS patterns shown in FIGS. 14 to 19 will first bedescribed and the CSI-RS patterns shown in FIGS. 20 to 36 will then bedescribed.

As shown in FIGS. 14 to 19, the CSI-RSs may be arranged on two OFDMsymbols in one subframe and may be arranged at four subcarrier locationsin one OFDM symbol. Thus, the CSI-RSs may be arranged on a total ofeight REs.

Both of the two OFDM symbols on which the CSI-RSs are arranged may beOFDM symbols on which the CRSs and DRSs are not arranged (the case ofthe normal CP of FIGS. 15( a) and 15(b) and FIG. 18( a)).

Alternatively, four REs in one of the two OFDM symbols on which theCSI-RSs are arranged may be arranged in a manner of reusing the REs onwhich the existing CRSs are arranged (the case of the extended CP ofFIGS. 14( a), 14(b), 14(c), 15(c), 16(b), 17(a), 17(b) and 18(a), in thecase of the normal CP of FIGS. 18( b), and 18(c)). In particular, inFIG. 17( b), if the REs to which the CRSs for the antenna port indexes 2and 3 are allocated are reused for the CSI-RSs, up to the CRSssupporting two transmission antennas may be specified.

Alternatively, eight REs in the two OFDM symbols on which the CSI-RSsare arranged may be arranged in a manner of reusing the REs on which theexisting CRSs are arranged (the case of the extended CP of FIGS. 16( a),16(b), 17(c) and 18(b)).

The embodiments of FIGS. 19( a) and 19(b) correspond to modifiedexamples in which the frequency locations are shifted by one subcarrieror two subcarriers in the CSI-RS pattern of FIG. 14( a).

Next, the CSI-RS patterns of FIGS. 20 to 36 will be described. In theembodiments of FIGS. 20 to 36, the CSI-RSs may be arranged on two OFDMsymbols in one subframe and may be arranged at four subcarrier locationsin one OFDM symbol. Thus, the CSI-RSs may be arranged on a total ofeight REs.

Both of the two OFDM symbols on which the CSI-RSs are arranged may beOFDM symbols on which the CRSs and DRSs are not arranged (the case ofthe normal CP of FIGS. 29( a) and the case of the normal CP of FIG. 36(a)).

Alternatively, one of the two OFDM symbols on which the CSI-RSs arearranged is an OFDM symbol on which the CRSs and DRSs are not arrangedand four REs on the remaining one OFDM symbol may be arranged in amanner of reusing the REs on which the existing CRSs are arranged (thecase of the normal CP of FIGS. 21( a) and 24(a), the case of the normalCP of FIG. 25( a), the case of the normal CP of FIG. 26( a), the case ofthe extended CP of FIGS. 28( a), 28(b), 28(c) and 29(a), the case of thenormal CP of FIGS. 29( b), 30(c) and 32(a), the case of the normal CP ofFIG. 33( c) and the case of the extended CP of FIG. 36( a)).

Alternatively, one of the two OFDM symbols on which the CSI-RSs arearranged is an OFDM symbol on which the CRSs and DRSs are not arrangedand the remaining OFDM symbol may be an OFDM symbol on which DRSs arearranged (the case of the normal CP of FIG. 22( c), the case of thenormal CP of FIG. 26( b), the case of the normal CP of FIGS. 29( c) and34(a), the case of the normal CP of FIG. 36( b) and the case of thenormal CP of FIG. 36( c)).

Alternatively, one of the two OFDM symbols on which the CSI-RSs arearranged is an OFDM symbol on which the DRSs are arranged and four REson the remaining one OFDM symbol may be arranged in a manner of reusingthe RES on which the existing CRSs are arranged (the case of theextended CP of FIGS. 20( a), 21(c), 22(a), 22(b) and 22(c), the case ofthe extended CP of FIGS. 23( a), 24(c), 25(c) and 26(b), the case of theextended CP of FIGS. 26( c), 31(b), 32(c), 33(b) and 34(a), the case ofthe extended CP of FIGS. 34( b) and 36(b), and the case of the extendedCP of FIG. 36( c)).

Alternatively, both of the OFDM symbols on which the CRI-RSs arearranged may be OFDM symbols on which the DRSs are arranged (FIG. 23(b)).

Alternatively, eight REs on the two OFDM symbols on which the CSI-RSsare arranged may be arranged in a manner of reusing the RES on which theexisting CRSs are arranged (the case of the extended CP of FIGS. 20( b),20(c), 21(b), 23(c) and 24(a), the case of the extended CP of FIGS. 24(b) and 25(a), the case of the extended CP of FIGS. 25( b) and 26(a), thecase of the extended CP of FIGS. 30( a), 30(b) and 30(c), the case ofthe extended P of FIGS. 31( a), 31(c) and 32(a), and the case of theextended CP of FIGS. 32( b), 33(a) and 33(c)).

The embodiments of FIGS. 27( a) and 27(b) correspond to modifiedexamples in which the frequency locations are shifted from the CSI-RSpattern of FIG. 22( c) by one subcarrier or two subcarriers. Theembodiments of FIGS. 35( a) and 35(b) correspond to modified examples inwhich the frequency locations are shifted from the CSI-RS pattern ofFIG. 34( a) by one subcarrier or two subcarriers.

Embodiment 3

Embodiment 3 relates to a method of multiplexing CSI-RSs for a pluralityof antenna ports based on the various examples of the locations of theCSI-RSs (that is, the locations of the OFDM symbols on which the CSI-RSsare arranged) on the time axis described in the above-describedEmbodiments 1 and 2. The frequency locations of the CSI-RSs described inthe embodiments of FIGS. 37 and 38 are applicable to both the normal CPand the extended CP of Embodiments 1 and 2 for the locations of theCSI-RSS on the time axis.

As shown in FIG. 37, in Embodiment 3, it is assumed that eight REs (twoOFDM symbols and four REs of one OFDM symbol) are used for CSI-RSs inone RB (one subframe of the time domain×12 subcarriers of the frequencydomain). In Pattern 1 of FIG. 37( a), the locations of two OFDM symbolsmay correspond to various OFDM symbol locations proposed in theabove-described Embodiments 1 and 2. The CSI-RSs located on therespective OFDM symbols may be arranged at an interval of 3 REs. Inaddition, the locations of the CSI-RSs shown in Pattern 1 of FIG. 37( a)may be shifted by 1 RE (Pattern 2) or 2 REs (Pattern 3) in the frequencydomain.

FIG. 37( b) shows a detailed example in which frequency shifting isperformed with respect to the pattern in which the CSI-RSs are arrangedusing the CDM-T scheme as shown in FIG. 38( b). FIG. 37( c) shows anexample in which the pattern in which the CSI-RSs are arranged using theCDM-T scheme as shown in FIG. 38( b) is arranged on an RB. Morespecifically, two OFDM symbols may be arranged on OFDM symbol indexes 9and 10 in one RB in the case of the normal CP and may be arranged onOFDM symbol indexes 7 and 8 in the case of the extended CP.

In order to acquire channels of N transmission antennas using theCSI-RSs, independent frequency/time/code resources for the N antennaports may be allocated. That is, the CSI-RSs for the N antenna ports maybe multiplexed using the FDM/TDM/CDM scheme.

FIG. 38 shows various embodiments of the method of multiplexing CSI-RSs.The locations of the two OFDM symbols on which the CSI-RSs are arrangedas shown in FIG. 38 may correspond to various OFDM symbol locationsproposed in the above-described Embodiments 1 and 2. Although omittedfor clarity of description, even in the various embodiments describedwith reference to FIG. 38, similarly to Patterns 1 to 3 of FIG. 37( a),the locations of the REs to which the CSI-RSs are mapped may befrequency-shifted by 1 RE or 2 REs.

In the embodiment of FIG. 38( a), the CSI-RSs (A, B, C, D, E, F, G andH) for eight antenna ports may be mapped to eight REs, respectively. Theantenna ports A, B, C and D may be distinguished using the FDM scheme.In addition, the antenna ports E, F, G and H may be distinguished usingthe FDM scheme. A first antenna port group (A, B, C and D) and a secondantenna port group (E, F, G and H) may be distinguished using the TDMscheme.

In the embodiment of FIG. 38( b), two of the eight antenna ports may bemultiplexed using the CDM-T scheme. For example, the antenna ports 0 and1 may be spread in the time domain using an orthogonal code having alength of 2 (e.g., Walsh code, DFT code, random code, etc.). That is,the CSI-RSs for the antenna port indexes 0 and 1 are spread by coderesources in the time domain and are arranged on the same REs (A of thefirst OFDM symbol and A of the second OFDM symbol). Although the antennaports 0 and 1 are described as an example, the multiplexing method usingthe CDM-T scheme is applicable to any two of the eight antenna ports.The eight antenna ports may be grouped into four antenna groups (thatis, A, B, C and D) each including two antenna ports and the four antennagroups may be distinguished using the FDM scheme.

In the embodiment of FIG. 38( c), two of the eight antenna ports may bemultiplexed using the CDM-F scheme. For example, the antenna ports 0 and1 may be spread in the frequency domain using an orthogonal code havinga length of 2 (e.g., Walsh code, DFT code, random code, etc.). That is,the CSI-RSs for the antenna port indexes 0 and 1 are spread by coderesources in the frequency domain and are arranged on the same REs (Aand A of the first OFDM symbol). Although the antenna ports 0 and 1 aredescribed as an example, the multiplexing method using the CDM-F schemeis applicable to any two of the eight antenna ports. The eight antennaports may be grouped into four antenna groups (that is, A, B, C and D)each including two antenna ports and the four antenna groups may bedistinguished using the FDM/TDM scheme.

FIG. 38( d) shows an embodiment in which two of the eight antenna portsare multiplexed using the CDM-F scheme similarly to FIG. 38( c) and fourantenna groups are multiplexed using the FDM/TDM scheme, which isdifferent from the embodiment of FIG. 38( c) in that the REs (e.g., Aand A) to which the CDM-F scheme is applied are located at an intervalof 6 REs, not at an interval of 3 REs).

In the embodiment of FIG. 38( e), four of the eight antenna ports may bemultiplexed using the CDM-F scheme. For example, the antenna ports 0, 1,2 and 3 may be spread in the frequency domain using an orthogonal codehaving a length of 4 (e.g., Walsh code, DFT code, random code, etc.).That is, the CSI-RSs for the antenna port indexes 0, 1, 2 and 3 arespread by code resources in the frequency domain and are arranged on thesame REs (A, A, A and A of the first OFDM symbol). Although the antennaports 0, 1, 2 and 3 are described as an example, the multiplexing methodusing the CDM-F scheme is applicable to any four of the eight antennaports. The eight antenna ports may be grouped into two antenna groups(that is, A and B) each including four antenna ports and the two antennagroups may be distinguished using the TDM scheme.

In the embodiment of FIG. 38( f), four of the eight antenna ports may bemultiplexed using the CDM scheme. For example, the antenna ports 0, 1, 2and 3 may be spread in the time domain and the frequency domain using anorthogonal code (e.g., Walsh code, DFT code, random code, etc.). Thatis, the CSI-RSs for the antenna port indexes 0, 1, 2 and 3 are spread bycode resources in the time domain and the frequency domain and arearranged on the same REs (A, A, A and A of the first and second OFDMsymbols). Although the antenna ports 0, 1, 2 and 3 are described as anexample, the multiplexing method using the CDM scheme is applicable toany four of the eight antenna ports. The eight antenna ports may begrouped into two antenna groups (that is, A and B) each including fourantenna ports and the two antenna groups may be distinguished using theFDM/TDM scheme.

In the embodiment of FIG. 38( g), four of the eight antenna ports may bemultiplexed using the CDM-T/F scheme. For example, the antenna ports 0,1, 2 and 3 may be spread in the time domain and the frequency domainusing an orthogonal code (e.g., Walsh code, DFT code, random code,etc.). That is, the CSI-RSs for the antenna port indexes 0, 1, 2 and 3are spread by code resources in the time domain and the frequency domainand are arranged on the same REs (A, A, A and A of the first and secondOFDM symbols). Although the antenna ports 0, 1, 2 and 3 are described asan example, the multiplexing method using the CDM scheme is applicableto any four of the eight antenna ports. The eight antenna ports may begrouped into two antenna groups (that is, A and B) each including fourantenna ports and the two antenna groups may be distinguished using theFDM scheme.

In the embodiment of FIG. 38( h), the eight antenna ports may bemultiplexed using the CDM scheme. For example, the antenna ports 0, 1,2, 3, 4, 5, 6 and 7 may be spread in the time domain and the frequencydomain using an orthogonal code (e.g., Walsh code, DFT code, randomcode, etc.). That is, the CSI-RSs for the antenna port indexes 0, 1, 2,3, 4, 5, 6 and 7 are spread by code resources in the time domain and thefrequency domain and are arranged on the same REs (A, A, A, A, A, A, Aand A of the first and second OFDM symbols). Alternatively, the CSI-RSsfor the antenna ports may be spread in the frequency domain and the samesignal may be transmitted in the time domain.

In the above-described embodiments, the CSI-RS patterns defined foreight antenna ports may be used for CSI-RSs for four antenna ports orCSI-RSs for two antenna ports as the same patterns (that is, the CSI-RSsare arranged at the same RE locations). At this time, all REs of theCSI-RS pattern for eight antenna ports may be used or a subset of someof the antenna ports may be used. This property may be called a nestedproperty.

The CSI-RS pattern in the case of the extended CP may be defined in astate in which CRSs for two transmission antennas are arranged. That is,the above-described various CSI-RS patterns are applicable to theextended CP on the assumption that a CRS pattern indicated by R2 and R3(third and fourth antenna ports) is not used and only a CRS patternindicated by R0 and R1 (first and second antenna ports) is used in FIG.6. The UE may recognize that the base station uses two antenna ports viaa previously defined PBCH.

Alternatively, in a state in which the UE recognizes that the basestation uses four transmission antennas via a PBCH, CRSs indicated by R2and R3 of a second slot of one subframe of FIG. 6 may not be used andsymbols corresponding thereto may be used as symbols for transmittingCSI-RSs. In addition, CRSs indicated by R2 and R3 of a second slot ofone subframe may not be used and RE locations of the corresponding CRSsmay be reused for CSI-RSs.

Embodiment 4

Embodiment 4 relates to additional embodiments of a CSI-RS pattern.

FIG. 39 is a diagram showing a pattern of other RSs to be considered inorder to determine a CSI-RS pattern. In FIG. 39, S denotes acell-specific RS (that is, CRS), U denotes a UE-specific RS (that is,DRS) defined in the conventional LTE standard (e.g., LTE release 8), andD denotes a UE-specific RS (DRS) which is newly defined in the LTEstandard (e.g., release 9 and 10).

Locations of various types of RSs allocated to one RE (one subframe inthe time domain×12 subcarriers in the frequency domain) will bedescribed with reference to FIG. 39.

FIG. 39( a) shows the case of the normal CP. CRSs S are located on OFDMsymbol indexes 0, 1, 4, 7, 8 and 11 and subcarrier indexes 0, 3, 6 and9. Existing LTE DRSs U are located on OFDM symbol indexes 3, 6, 9 and 12and subcarrier indexes 0, 4 and 8 or subcarrier indexes 2, 6 and 10.DRSs D are located on OFDM symbol indexes 5, 6, 12 and 13 and subcarrierindexes 0, 1, 5, 6, 10 and 11.

FIG. 39( b) shows the case of the extended CP. CRSs S are located onOFDM symbol indexes 0, 1, 3, 6, 7 and 9 and subcarrier indexes 0, 3, 6and 9. Existing LTE DRSs U are located on OFDM symbol indexes 4, 7 and10 and subcarrier indexes 0, 3, 6 and 9 or subcarrier indexes 2, 5, 8and 11. DRSs D are located on OFDM symbol indexes 4, 5, 10 and 11 andsubcarrier indexes 1, 2, 4, 5, 6, 7, 8, 10 and 11.

CSI-RSs may be defined for eight transmission antennas, fourtransmission antennas and two transmission antennas. CSI-RS patterns forfour transmission antennas and two transmission antennas may be definedas a set or a subset of CSI-RS patterns for eight transmission antennas.That is, a nested property may be satisfied.

In determination of the locations of the CSI-RSs in the time domain,OFDM symbols including CRSs, DRSs (including DRSs defined in LTE release8, 9 and 10, that is, U and D of FIG. 39) and PDCCH may be excluded.Thus, only the OFDM symbol index 10 may be used for CSI-RS allocation inthe case of the normal CP and only the OFDM symbol index 8 may be usedfor CSI-RS allocation in the case of the extended CP.

Hereinafter, the locations of the CSI-RSs allocated to one OFDM symbol(the OFDM symbol index 10) in the frequency domain in the case of thenormal CP will be described with reference to FIG. 40. For clarity ofdescription, although only the normal CP is described, the descriptionof FIG. 40 is equally applicable to the locations of the CSI-RSsallocated to one OFDM symbol (the OFDM symbol index 8) in the frequencydomain in the case of the extended CP.

First, a CSI-RS structure of an FDM scheme will be described. The FDMscheme is a scheme for distinguishing CSI-RSs for transmission antennasusing frequency resources in the case of two transmission antennas, fourtransmission antennas or eight transmission antennas. The structuresupporting eight antennas includes a structure in which eight REs areconsecutively arranged (Pattern 1 of FIG. 40( a), a structure in whichunits of two consecutive REs are arranged at a predetermined interval(Pattern 2 of FIG. 40( a) and a structure in which units of fourconsecutive REs are arranged at a predetermined interval (Pattern 3 ofFIG. 40( a)). If eight consecutive REs are arranged, a CSI-RS for oneantenna port corresponds to each RE.

Next, a CSI-RS structure of a CDM-FDM scheme will be described. EightREs are divided into pairs of two. That is, pairs of A-A, B-B, C-C andD-D are formed. An orthogonal code having a length of 2 may be allocatedto one pair to distinguish between two antenna ports. At this time, inorder to distinguish between pairs, frequency resources may be used. Asthe CSI-RS structure for the case in which two REs forming a pair arearranged at a certain interval, Patterns 1 to 3 of FIG. 40( b) may beconsidered. Alternatively, as the CSI-RS structure for the case in whichtwo REs forming a pair may be located on consecutive subcarriers,Patterns 1 to 3 of FIG. 40( c) may be considered.

Indicators A, B, C and D of the CSI-RSs shown in FIG. 40 indicateantenna ports (antenna ports 0 to 7) or antenna groups and exemplarycorrespondence thereof is shown in Table 1. The present invention is notlimited to the correspondence shown in Table 1 and the order of A, B, Cand D may be arbitrarily changed to correspond to antenna ports orantenna groups.

TABLE 1 Method Method Method Method Method Method 1 2 3 4 5 6 8Tx 8Tx4Tx 4Tx 2Tx 2Tx A 0, 1 0, 4 0, 1 0 0, 1 0 B 2, 3 1, 5 2, 3 1 1 C 4, 5 2,6 2 D 6, 7 3, 7 3

In determination of the locations of the CSI-RSs arranged in the timedomain, OFDM symbols including CRSs, DRSs (including DRSs defined in LTErelease 9 and 10, that is, D of FIG. 39) and PDCCH may be excluded.Thus, unlike the embodiment described with reference to FIG. 40, apattern in which CSI-RSs are arranged on OFDM symbols, on which DRSs (Uof FIG. 39) defined in the conventional LTE are located, may beconsidered. In the case of the normal CP, OFDM symbol indexes 3, 9 and10 may be used for CSI-RS allocation.

A CSI-RS pattern in the case of the normal CP will be described withreference to FIG. 41. CSI-RSs may be located on OFDM symbol indexes 9and 10. The CSI-RSs may be located on six subcarriers in the frequencydomain at an interval of two subcarriers. At this time, the CSI-RSs maynot be arranged at subcarrier locations (subcarrier indexes 0, 4 and 8)in which DRSs U defined in the conventional LTE system are present. Thatis, CSI-RSs may be arranged on 12 REs in two OFDM symbols with thepattern shown in FIG. 41( a).

In FIG. 41( a), in order to distinguish between CSI-RSs for the channelsof eight antenna ports, eight distinguished resources (time, frequencyand/or code resources) must be used. Eight antenna ports may bemultiplexed through two distinguished time resources, six distinguishedfrequency resources and an orthogonal code. For example, an orthogonalcode may be allocated to contiguous time resources (contiguous OFDMsymbols) to distinguish between two antenna ports and four orthogonalfrequency resources may be allocated to distinguish between a total ofeight antenna ports. At this time, any four frequency resources may beselected from among the six frequency resources shown in FIG. 41( a).The number of cases of selecting any four frequency resources from amongthe six frequency resources is 360 (6p4=360) and three patterns may beused for three cells. That is, frequency-shifted patterns may be set tobe respectively used in three cells.

FIG. 41( b) shows an example of multiplexing CSI-RSs using fourfrequency resources, two time resources and two orthogonal coderesources. In FIG. 41( b), two antenna ports may be distinguishedthrough orthogonal code at the A-A location. In Patterns 1 to 3 of FIG.41( b), frequency shifting is performed at an interval of 2 subcarriers.Each pattern may be selected and used according to various criteria. Forexample, one pattern may be selected according to the elapse of time.

FIGS. 41( c) to 41(e) show various modified examples of selecting fourfrequency resources from among six frequency resources. In theembodiments of FIGS. 41( c) to 41(e), three frequency-shifted patternsmay be defined and each pattern may be selected according to variouscriteria.

In determination of the locations of the CSI-RSs, the CSI-RSs may bearranged on REs other than the locations of the DRSs in the OFDM symbolon which the DRSs (including DRSs defined in LTE release 8, 9 and 10,that is, U and D of FIG. 39) are located. Then, in the case of thenormal CP, OFDM symbol indexes 3, 5, 6, 9, 10, 12 and 13 may be used.Although the CSI-RSs are described as being arranged on the OFDM symbolindexes 9 and 10 in the above-described embodiments, the case in whichthe CSI-RSs are arranged on the OFDM symbol indexes 5, 6, 12 and 13, onwhich the DRSs D are located, in the case of the normal CP will now bedescribed.

FIG. 42( a) shows a CSI-RS pattern according to frequency shiftingVshift of the DRSs (U of FIG. 39) defined in the conventional LTEstandard (LTE release 8). The DRSs U are frequency-shifted by 0, 1 or 2subcarriers based on a cell ID. The locations of the DRSs (that is, D ofFIG. 39) defined in LTE release 9 and 10 are fixed.

The CSI-RSs are located in the OFDM symbol on which the DRSs D arearranged and may be arranged on REs other than the frequency locationson which the DRSs (U and D) are arranged. In consideration of CDM, twocontiguous OFDM symbols may be paired to configure a CSI-RS. At thistime, two or four frequency locations may be selected in one OFDMsymbol.

If two frequency locations are selected in one OFDM symbol, the CSI-RSsmay be arranged at two frequency locations of each of the OFDM symbolindexes 5, 6, 12 and 13, an example of which is shown in FIG. 42( b).FIGS. 43( a) and 43(b) show modified examples in which the frequencylocations are changed in the CSI-RS pattern.

If two frequency locations are selected in one OFDM symbol in order toarrange the CSI-RSs, a total of four OFDM symbols is used for the CSI-RSpattern. At this time, the antenna ports mapped to the REs set in theCSI-RS pattern may be changed in certain frequency units. For example,in an odd-numbered RB, antenna ports 0, 1, 2 and 3 may be mapped to twoOFDM symbols (OFDM symbol indexes 5 and 6) from the front portion andantenna ports 4, 5, 6 and 7 may be mapped to two OFDM symbols (OFDMsymbol indexes 12 and 13) from the rear portion. In an even-numbered RB,antenna ports 4, 5, 6 and 7 may be mapped to two OFDM symbols (OFDMsymbol indexes 5 and 6) from the front portion and antenna ports 0, 1, 2and 3 may be mapped to two OFDM symbols (OFDM symbol indexes 12 and 13)from the rear portion. The mapped antenna port indexes and the frequencyunit for swapping the antenna ports are exemplary and other antenna portmapping relationships and swapping frequency units may be used.

If four frequency locations are selected in one OFDM symbol, anembodiment in which the CSI-RSs are arranged on the OFDM symbols 5 and 6is shown in FIG. 44( a). If four frequency locations are selected in oneOFDM symbol, an embodiment in which the CSI-RSs are arranged on the OFDMsymbols 12 and 13 is shown in FIG. 44( b).

In the above-described embodiments, AA, BB, CC and DD mean units towhich the orthogonal code is applied. Walsh code, etc. may be used asthe orthogonal code. The antenna ports or antenna port groups may bemapped to A to D of the drawing. A mapping relationship in the case ofeight transmission antennas, four transmission antennas and twotransmission antennas is shown in Table 1.

In the above-described embodiments, the CSI-RSs may be frequency-shiftedusing the same method as the CRSs. That is, the CSI-RSs may befrequency-shifted on a per cell basis.

Next, the locations of the CSI-RSs in the case of the extended CP willbe described.

In determination of the locations of the CSI-RSs in the time domain, ifOFDM symbols including CRSs, DRSs (including DRSs defined in the LTErelease 9 and 10, that is, D of FIG. 39) and PDCCH may be excluded, onlythe OFDM symbol index 8 may be used for CSI-RS allocation in the case ofthe extended CP. In order to arrange the CSI-RSs on two consecutive OFDMsymbols, the OFDM symbol indexes 7 and 8 may be used.

FIG. 45( a) shows a pattern in which the CSI-RSs are arranged in case ofusing the OFDM symbol indexes 7 and 8. The CSI-RSs may be arranged onREs on which CRSs or DRSs U are not located. The CSI-RSs may befrequency-shifted on a per cell basis, as in the CRSs and the DRSs Udefined in LTE release 8. A to D of FIG. 45( a) may be mapped to antennaports or antenna port groups and correspondence thereof is shown inTable 1.

Alternatively, in the case of the extended CP, the number oftransmission antennas supported by the CRSs may be restricted to 2 andonly the CRSs (R0 and R1 of FIG. 6) for two transmission antennas may beset to be allocated. In this case, the CSI-RSs may be allocated byreusing the REs to which the CRSs (R2 and R3 of FIG. 6) are allocated.

In determination of the locations of the CSI-RSs, the CSI-RSs may bearranged on the OFDM symbol on which the DRSs (including DRSs defined inLTE release 8, 9 and 10, that is, U and D of FIG. 39) are located. Thus,in the case of the extended CP, the OFDM symbol indexes 4, 5, 10 and 11may be used.

Among the OFDM symbol indexes 4, 5, 10 and 11, two or four OFDM symbolsmay be used for CSI-RS allocation.

If two OFDM symbols are selected, the CSI-RSs may be arranged on fourfrequency locations in one OFDM symbol. FIG. 45( b) shows the case inwhich the OFDM symbol indexes 4 and 5 are selected FIG. 45( c) shows thecase in which the OFDM symbol indexes 10 and 11 are selected. In FIGS.45( b) and 45(c), four frequency locations in one OFDM symbol may beallocated for CSI-RSs and an interval thereof may be set to twosubcarriers. The CSI-RS patterns of FIGS. 45( b) and 45(c) may befrequency-shifted by one subcarrier. The frequency-shifted CSI-RSpattern may be used on a per cell basis. Since transmission of DRSs D isrequired in FIGS. 45( b) and 45(c), the number of frequency-shiftedpatterns may be restricted to 2 such that the DRSs are transmitted atleast four frequency locations in one OFDM symbol.

If four OFDM symbols are selected, the CSI-RSs may be arranged on twofrequency locations in one OFDM symbol. FIG. 45( d) shows an example ofa CSI-RS pattern in which the four CSI-RSs are arranged on the OFDMsymbol indexes 4, 5, 10 and 11 and FIG. 45( e) corresponds to anembodiment obtained by frequency-shifting the pattern of FIG. 45( d).

In FIG. 45, the antenna ports or the antenna port groups may be mappedto A to D of the drawing. A mapping relationship in the case of eighttransmission antennas, four transmission antennas and two transmissionantennas is shown in Table 1.

Embodiment 5

Embodiment 5 relates to a detailed example of a CSI-RS pattern to whichthe above-described Embodiments 1 to 4 are applicable.

FIG. 46( a) shows an example in which the CSI-RSs are arranged on atotal of eight REs in two OFDM symbols. Eight REs for the CSI-RSs may bearranged on two OFDM symbols, that is, four REs may be arranged on oneOFDM symbol. In the four REs in one OFDM symbol, two REs may beconsecutively arranged and the remaining two REs may be arranged to beseparated therefrom by four subcarriers.

If the CSI-RSs are transmitted with a duty cycle of 1, all the CSI-RSsfor eight transmission antennas may be allocated within one subframe. Ifthe CSI-RSs for the antenna port indexes 0 to 7 are transmitted, forexample, the antenna port indexes 0 and 1 may be allocated to the CSI-RS1 of FIG. 46( a) using the CDM-T scheme, the antenna port indexes 2 and3 may be allocated to the CSI-RS 2 using the CDM-T scheme, the antennaport indexes 4 and 5 may be allocated to the CSI-RS 3 using the CDM-Tscheme, and the antenna port indexes 6 and 7 may be allocated to theCSI-RS 4 using the CDM-T scheme.

As shown in FIGS. 46( b) and 46(c), the CSI-RS pattern of FIG. 46( a)may be frequency-shifted. This may indicate that the start point of thefrequency location of the CSI-RS pattern shown in FIG. 46( a) may bemoved according to an offset. For example, such an offset value may havea value of 1 to 8 subcarriers and may be determined on a per cell orcell group basis. FIG. 46( b) shows an embodiment obtained by frequencyshifting the pattern of FIG. 46( a) by two subcarriers and FIG. 46( c)shows an embodiment obtained by frequency shifting the pattern of FIG.46( b) by two subcarriers.

The CSI-RS pattern may have a cell-specific frequency shift value. Forexample, the cell-specific frequency shifting value may be twosubcarriers. That is, in three cells, the CSI-RSs may be arranged so asnot to overlap the frequency locations of the CSI-RSs on the same OFDMsymbol. For example, a first cell may use the CSI-RS pattern of FIG. 46(a), a second cell may use the CSI-RS pattern of FIG. 46( b) and a thirdcell may use the CSI-RS pattern of FIG. 46( c).

The OFDM symbols to which the CSI-RS pattern shown in FIG. 46( a) isallocated may correspond to various time locations described in theabove embodiments. For example, the OFDM symbols to which the CSI-RSpattern of FIG. 46( a) may be allocated to the OFDM symbol indexes 9 and10 in the case of the normal CP (FIG. 47( a)) or the OFDM symbol indexes8 and 10 in the case of the normal CP (FIG. 47( b)). As described above,CSI-RS patterns obtained by frequency-shifting the CSI-RS pattern ofFIG. 47 by an interval of two subcarriers may be used by other cells andthe CSI-RS patterns may be used so as not to overlap in the three cells.

Next, FIG. 48( a) shows another example in which the CSI-RS are arrangedon a total of eight REs in two OFDM symbols. Eight REs for the CSI-RSsmay be arranged on two OFDM symbols, that is, four REs may be arrangedon one OFDM symbol. The four REs in one OFDM symbol are arranged to beseparated from each other by 2 subcarriers.

If the CSI-RSs are transmitted with a duty cycle of 1, all the CSI-RSsfor eight transmission antennas may be allocated within one subframe. Ifthe CSI-RSs for the antenna port indexes 0 to 7 are transmitted, forexample, the antenna port indexes 0 and 1 may be allocated to the CSI-RS1 of FIG. 46( a) using the CDM-T scheme, the antenna port indexes 2 and3 may be allocated to the CSI-RS 2 using the CDM-T scheme, the antennaport indexes 4 and 5 may be allocated to the CSI-RS 3 using the CDM-Tscheme, and the antenna port indexes 6 and 7 may be allocated to theCSI-RS 4 using the CDM-T scheme.

As shown in FIGS. 48( b) and 48(c), the CSI-RS pattern of FIG. 48( a)may be frequency-shifted. This may indicate that the start point of thefrequency location of the CSI-RS pattern shown in FIG. 48( a) may bemoved according to an offset. For example, such an offset value may havea value of 1 to 8 subcarriers and may be determined on a per cell orcell group basis. FIG. 48( b) shows an embodiment obtained by frequencyshifting the pattern of FIG. 48( a) by one subcarrier and FIG. 48( c)shows an embodiment obtained by frequency shifting the pattern of FIG.48( b) by one subcarrier.

The CSI-RS pattern may have a cell-specific frequency shift value. Forexample, the cell-specific frequency shifting value may be onesubcarrier. That is, in three cells, the CSI-RSs may be arranged so asnot to overlap the frequency locations of the CSI-RSs on the same OFDMsymbol. For example, a first cell may use the CSI-RS pattern of FIG. 48(a), a second cell may use the CSI-RS pattern of FIG. 48( b) and a thirdcell may use the CSI-RS pattern of FIG. 48( c).

The OFDM symbols to which the CSI-RS pattern shown in FIG. 48( a) isallocated may correspond to various time locations described in theabove embodiments. For example, the OFDM symbols to which the CSI-RSpattern of FIG. 46( a) may be allocated to the OFDM symbol indexes 7 and8 in the case of the extended CP (FIG. 49). As described above, CSI-RSpatterns obtained by frequency-shifting the CSI-RS pattern of FIG. 48may be used by other cells and the CSI-RS patterns may be used so as notto overlap in the three cells.

FIG. 50 is a diagram showing the configuration of a wirelesscommunication system including a UE and a base station according to anexemplary embodiment of the present invention.

The base station (eNB) 5010 may include a reception (Rx) module 5011, atransmission (Tx) module 5012, a processor 5013, a memory 5014 and anantenna 5015. The Rx module 5011 may receive a variety of signals, data,information, etc. from a UE. The Tx module 5012 may transmit a varietyof signals, data, information, etc. to a UE. The processor 5013 may beconfigured to perform overall control of the base station 5010 includingthe Rx module 5011, the Tx module 5012, the memory 5014 and the antenna5015. The antenna 5015 may include a plurality of antennas.

The processor 5013 may map CSI-RSs for 8 or fewer antenna ports on adata region of a downlink subframe having the normal CP configurationaccording to a predetermined pattern and control the downlink subframeto which the CSI-RSs for the 8 or fewer antenna ports are mapped.

The processor 5013 serves to process information received by the UE andinformation to be transmitted to an external device. The memory 5014 maystore the processed information for a predetermined time and may bereplaced with a component such as a buffer (not shown).

The UE 5020 may include an Rx module 5021, a Tx module 5022, a processor5023 and a memory 5024. The Rx module 5021 may receive a variety ofsignals, data, information, etc. from a base station. The Tx module 5022may transmit a variety of signals, data, information, etc. to a basestation. The processor 5023 may be configured to perform overall controlof the base station 5020 including the Rx module 5021, the Tx module5022, the memory 5024 and the antenna 5025. The antenna 5025 may includea plurality of antennas.

The processor 5023 may receive CSI-RSs for 8 or fewer antenna portsmapped according to a predetermined pattern on a data region of adownlink subframe having the normal CP configuration and controlestimation of the channel using the CSI-RSs.

The processor 5033 serves to process information received by the UE andinformation to be transmitted to an external device. The memory 5034 maystore the processed information for a predetermined time and may bereplaced with a component such as a buffer (not shown).

Matters which are commonly applied to channel estimation in which thebase station 5010 transmits the CSI-RSs and the UE 5020 receives theCSI-RSs will be described.

The predetermined pattern according to which the CSI-RSs are mapped maybe determined in advance and may be shared by the base station 5010 andthe UE 5020. The predetermined pattern may be defined such that theCSI-RSs mapped for 8 or fewer antenna ports are mapped to two OFDMsymbols in the data region of the downlink subframe and are mapped toone or more of four subcarrier locations in one of the two OFDM symbols.The four subcarrier locations defined in the predetermined pattern mayinclude two consecutive subcarrier locations and two other consecutivesubcarrier locations separated therefrom by four subcarriers (see FIG.47).

When the processor maps the CSI-RSs according to the predeterminedpattern, the two OFDM symbols may be OFDM symbol indexes 5 and 6, OFDMsymbol indexes 9 and 10, OFDM symbol indexes 12 and 13 or OFDM symbolindexes 8 and 10. If the two OFDM symbol are OFDM symbol indexes 5 and 6or OFDM symbol indexes 12 and 13, the four subcarrier locations aresubcarrier indexes 2, 3, 8 and 9 and, if the two OFDM symbols are OFDMsymbol indexes 9 and 10 or OFDM symbol indexes 8 and 10, the foursubcarrier locations may be subcarrier indexes 0, 1, 6 and 7, subcarrierindexes 2, 3, 8 and 9 or subcarrier indexes 4, 5, 10 and 11 (see FIGS.44 and 47).

When the processor maps the CSI-RSs according to the predeterminedpattern, the four subcarrier locations may be shifted by two subcarrierson a per cell or cell group basis (see FIG. 46). In addition, when theprocessor maps the CSI-RSs according to the predetermined pattern, theCSI-RSs may be subjected to CDM using orthogonal code over the two OFDMsymbols (see FIG. 46). When the processor maps the CSI-RSs according tothe predetermined pattern, if the number of antenna ports of the basestation is 2 or 4, the CSI-RSs may be mapped to some of the locationsdefined in the predetermined pattern (the above-described nestedproperty).

The embodiments of the present invention can be implemented by a varietyof means, for example, hardware, firmware, software, or a combination ofthem.

In the case of implementing the present invention by hardware, thepresent invention can be implemented with application specificintegrated circuits (ASICs), Digital signal processors (DSPs), digitalsignal processing devices (DSPDs), programmable logic devices (PLDs),field programmable gate arrays (FPGAs), a processor, a controller, amicrocontroller, a microprocessor, etc.

If operations or functions of the present invention are implemented byfirmware or software, the present invention can be implemented in avariety of formats, for example, modules, procedures, functions, etc.The software code may be stored in a memory unit so that it can bedriven by a processor. The memory unit is located inside or outside ofthe processor, so that it can communicate with the aforementionedprocessor via a variety of well-known parts.

The detailed description of the exemplary embodiments of the presentinvention has been given to enable those skilled in the art to implementand practice the invention. Although the invention has been describedwith reference to the exemplary embodiments, those skilled in the artwill appreciate that various modifications and variations can be made inthe present invention without departing from the spirit or scope of theinvention described in the appended claims. For example, those skilledin the art may use each construction described in the above embodimentsin combination with each other. Accordingly, the invention should not belimited to the specific embodiments described herein, but should beaccorded the broadest scope consistent with the principles and novelfeatures disclosed herein.

Those skilled in the art will appreciate that the present invention maybe carried out in other specific ways than those set forth hereinwithout departing from the spirit and essential characteristics of thepresent invention. The above exemplary embodiments are therefore to beconstrued in all aspects as illustrative and not restrictive. The scopeof the invention should be determined by the appended claims and theirlegal equivalents, not by the above description, and all changes comingwithin the meaning and equivalency range of the appended claims areintended to be embraced therein. Moreover, it will be apparent that someclaims referring to specific claims may be combined with another claimsreferring to the other claims other than the specific claims toconstitute the embodiment or add new claims by means of amendment afterthe application is filed.

INDUSTRIAL APPLICABILITY

The above-described embodiments of the present invention are applicableto various mobile communication systems.

1. A method for transmitting reference signals for 8 or fewer antennaports at a base station, the method comprising: mapping some of commonreference signals (CRSs) for four or fewer antenna ports to a downlinksubframe including a first slot and a second slot and having a normalcyclic prefix (CP); mapping channel state information-reference signals(CSI-RSs) for the 8 or fewer antenna ports to the downlink subframeaccording to a predetermined pattern; and transmitting the downlinksubframe to which the CRSs and the CSI-RSs are mapped, wherein thepredetermined pattern defines the CSI-RSs for the 8 or fewer antennaports to be mapped to two orthogonal frequency division multiplexing(OFDM) symbols in a data region of the downlink subframe, the two OFDMsymbols being separated by one OFDM symbol, and wherein some of the CRSsfor the 4 or fewer antenna ports are limited to CRSs for 2 or fewerantenna ports.
 2. The method according to claim 1, wherein the CRSs forthe 2 or fewer antenna ports are mapped to first and fifth OFDM symbolsof the first slot and first and fifth OFDM symbols of the second slot.3. The method according to claim 1, wherein the two OFDM symbols definedin the predetermined pattern for the CSI-RSs are second and fourth OFDMsymbols of the second slot.
 4. The method according to claim 1, whereinthe predetermined pattern defines the CSI-RSs for the 8 or fewer antennaports to be mapped to one or more of four subcarrier locations in eachof the two OFDM symbols, and wherein the four subcarrier locationsdefined in the predetermined pattern include two consecutive subcarrierlocations and two other subcarrier locations separated therefrom by 4subcarriers.
 5. The method according to claim 4, wherein the foursubcarrier locations defined in the predetermined pattern are shifted by2 subcarriers on a per cell or cell group basis.
 6. The method accordingto claim 4, wherein the two OFDM symbols are the second and fourth OFDMsymbols of the second slot, and wherein the four subcarrier locationsare subcarrier indexes 0, 1, 6 and 7, subcarrier indexes 2, 3, 8 and 9or subcarrier indexes 4, 5, 10 and
 11. 7. The method according to claim1, wherein, if the number of antenna ports of the base station is 2 or4, the CSI-RSs are mapped to some of the locations defined in thepredetermined pattern.
 8. The method according to claim 1, wherein theCSI-RSs for the 8 or fewer antenna ports are grouped into a total offour groups such that CSI-RSs for two antenna ports configure one group,wherein the CSI-RSs for two antennas of each of the four groups aremultiplexed at the same subcarrier location of the two OFDM symbolsusing a code division multiplexing (CDM) scheme, and wherein the fourgroups are multiplexed at different subcarrier locations using afrequency division multiplexing (FDM) scheme.
 9. A method of, at a userequipment, estimating a channel using channel stateinformation-reference signals (CSI-RSs) for 8 or fewer antenna portsfrom a base station, the method comprising: receiving a downlinksubframe including a first slot and a second slot and having a normalcyclic prefix (CP), to which some of common reference signals (CRSs) forfour or fewer antenna ports are mapped and to which channel stateinformation-reference signals (CSI-RSs) for the 8 or fewer antenna portsare mapped according to a predetermined pattern; and estimating thechannel using the CSI-RSs, wherein the predetermined pattern defines theCSI-RSs for the 8 or fewer antenna ports to be mapped to two orthogonalfrequency division multiplexing (OFDM) symbols in a data region of thedownlink subframe, the two OFDM symbols being separated by one OFDMsymbol, and wherein some of the CRSs for the 4 or fewer antenna portsare limited to CRSs for 2 or fewer antenna ports.
 10. The methodaccording to claim 9, wherein the CRSs for the 2 or fewer antenna portsare mapped to first and fifth OFDM symbols of the first slot and firstand fifth OFDM symbols of the second slot.
 11. The method according toclaim 9, wherein the two OFDM symbols defined in the predeterminedpattern for the CSI-RSs are second and fourth OFDM symbols of the secondslot.
 12. The method according to claim 9, wherein the predeterminedpattern defines the CSI-RSs for the 8 or fewer antenna ports to bemapped to one or more of four subcarrier locations in each of the twoOFDM symbols, and wherein the four subcarrier locations defined in thepredetermined pattern include two consecutive subcarrier locations andtwo other subcarrier locations separated therefrom by 4 subcarriers. 13.The method according to claim 12, wherein the four subcarrier locationsdefined in the predetermined pattern are shifted by 2 subcarriers on aper cell or cell group basis.
 14. The method according to claim 12,wherein the two OFDM symbols are the second and fourth OFDM symbols ofthe second slot, and wherein the four subcarrier locations aresubcarrier indexes 0, 1, 6 and 7, subcarrier indexes 2, 3, 8 and 9 orsubcarrier indexes 4, 5, 10 and
 11. 15. The method according to claim 9,wherein, if the number of antenna ports of the base station is 2 or 4,the CSI-RSs are mapped to some of the locations defined in thepredetermined pattern.
 16. The method according to claim 9, wherein theCSI-RSs for the 8 or fewer antenna ports are grouped into a total offour groups such that CSI-RSs for two antenna ports configure one group,wherein the CSI-RSs for two antennas of each of the four groups aremultiplexed at the same subcarrier location of the two OFDM symbolsusing a code division multiplexing (CDM) scheme, and wherein the fourgroups are multiplexed at different subcarrier locations using afrequency division multiplexing (FDM) scheme.
 17. A base station fortransmitting reference signals (RSs) for 8 or fewer antenna ports,comprising: a reception module configured to receive an uplink signalfrom a user equipment; a transmission module configured to transmit adownlink signal to the user equipment; and a processor configured tocontrol the base station including the reception module and thetransmission module, wherein the processor maps some of common referencesignals (CRSs) for four or fewer antenna ports to a downlink subframeincluding a first slot and a second slot and having a normal cyclicprefix (CP), maps channel state information-reference signals (CSI-RSs)for the 8 or fewer antenna ports to the downlink subframe according to apredetermined pattern, and controls transmission of the downlinksubframe to which the CRSs and the CSI-RSs are mapped, wherein thepredetermined pattern defines the CSI-RSs for the 8 or fewer antennaports to be mapped to two orthogonal frequency division multiplexing(OFDM) symbols in a data region of the downlink subframe, the two OFDMsymbols being separated by one OFDM symbol, and wherein some of the CRSsfor the 4 or fewer antenna ports are limited to CRSs for 2 or fewerantenna ports.
 18. A user equipment for estimating a channel usingchannel state information-reference signals (CSI-RSs) for 8 or fewerantenna ports from a base station, comprising: a reception moduleconfigured to receive a downlink signal from the base station; atransmission module configured to transmit an uplink signal to the basestation; and a processor configured to control the user equipmentincluding the reception module and the transmission module, wherein theprocessor receives a downlink subframe including a first slot and asecond slot and having a normal cyclic prefix (CP), to which some ofcommon reference signals (CRSs) for four or fewer antenna ports aremapped and to which channel state information-reference signals(CSI-RSs) for the 8 or fewer antenna ports are mapped according to apredetermined pattern, and controls estimation of the channel using theCSI-RSs, wherein the predetermined pattern defines the CSI-RSs for the 8or fewer antenna ports to be mapped to two orthogonal frequency divisionmultiplexing (OFDM) symbols in a data region of the downlink subframe,the two OFDM symbols being separated by one OFDM symbol, and whereinsome of the CRSs for the 4 or fewer antenna ports are limited to CRSsfor 2 or fewer antenna ports.